To quote "Spider-Man: Into the Spider-Verse": 
"Alright, people, let's do this one last time."
This September will be the last in which Major League active rosters can expand to include any members of the 40-man roster. Starting next year, all clubs will need to carry exactly 28 players from Sept. 1 until the end of the season. (Regular-season roster limits before Sept. 1 will go up from 25 to 26.) In other words, this could be the last time that waves of prospects in different organizations jump into a Major League season in progress all at once.
To prepare fans for this final big September, this edition of Toolshed looks at which players among MLB.com's Top 100 Prospects definitely will head up to the Majors, could squeak in and are long shots to get that look at The Show.

Already up

Bo Bichette (Blue Jays), Mitch Keller (Pirates), Dustin May (Dodgers), Isan Díaz (Marlins)

More than likely

Kyle Tucker, outfielder, Astros: Entering the season, the odds on Tucker playing in Houston were pretty good. He played a full season at Triple-A in 2018 and got 64 at-bats with the big club on top of that. Sure, he struggled in that time (hitting .141 with a .439 OPS), but no one doubted that the 22-year-old still had the talent to crack the Astros lineup at some point again the following season. Yet here we are. Tucker was passed over by the next big phenom in Yordan Alvarez, and after some early struggles, he ended up playing all 123 of his games this season with Round Rock. But the No. 13 overall prospect should still be a huge asset to Houston's playoff push this next month. Tucker became just the fifth player in Pacific Coast League history to reach 30 homers and 30 stolen bases in a season -- the last being Joc Pederson in 2014 -- and his combo of power and speed should be useful off the bench. He also played both corner outfield spots and even some first base this summer, so a few late-inning defensive replacements aren't out of the question, either. Let Tucker's long-term role be up for discussion in the offseason. For now, he's too good to be left at home when he's already on the 40-man.
Brendan McKay, left-handed pitcher/DH, Rays: McKay made eight starts for the Rays this summer but was sent down last week after giving up seven runs (three earned) in two innings on Aug. 19. Shortly after, he was placed on the injured list with shoulder fatigue, but Tampa Bay has already said it expects the stint to be a short one as nothing is structurally wrong. It's true that McKay was hitting a bit of a wall at the top level with a 5.55 ERA in 35 2/3 innings, but his four-pitch mix will come in handy, should he be fully healthy. His ceiling remains as a starter, but it's possible Tampa Bay -- which can't afford missteps in the AL Wild Card chase -- eases him back in through the bullpen when rosters expand.
Carter Kieboom, shortstop/second baseman, Nationals: The Kieboom Era in the nation's capital actually began in late April. It didn't last long. The Nats' top prospect went 5-for-39 (.128) with two homers and 16 strikeouts and made four errors in 11 games at shortstop as a fill-in for the injured Trea Turner and hasn't been back since. In the meantime, he became a PCL end-of-season All-Star and entered Thursday's play with a .305/.413/.501 line, 16 homers and a 126 wRC+ in 105 games for Fresno. His bat will play, even if it didn't the first time around, and it's not one the Wild Card-leading Nationals can afford to leave behind. In the best-case scenario, Kieboom shows enough to wrestle the second-base job away from Brian Dozier and Asdrúbal Cabrera. In the worst, he heads back to the PCL for his age-22 season, much like Tucker did this summer.
Kyle Wright, right-handed pitcher, Braves: Sensing a theme here? Wright -- a September callup last year -- opened the season with three starts for Atlanta and then returned to make another start in July and got shellacked during most of that time, giving up 18 earned runs, walking 13 and giving up four homers in 16 2/3 innings. Unlike some of the others above, Wright's time at Triple-A hasn't been rosy; the No. 36 overall prospect posted a 4.27 ERA with 107 strikeouts in 105 1/3 innings with Gwinnett. That said, Wright's stuff, which includes four above-average pitches when at their best, is still worth a look in shorter bullpen stints for the big club. His 122 innings between Triple-A and the Majors is still below his 144 from a season ago, so workload shouldn't be an issue. Showing Major League readiness and the ability to get batters out over longer stretches will be what matters.
Logan Allen, left-handed pitcher, Indians: The Tribe picked up Allen from the Padres in the three-team deal that sent Trevor Bauer to Cincinnati and Taylor Trammell to San Diego four days after he had just been optioned from the Major League roster himself. He made his Cleveland debut on Aug. 14, striking out three over 2 1/3 scoreless innings in relief. That's probably where he'll fit again in September with the Indians, who sit two games ahead of the A's for the first AL Wild Card spot. The No. 95 overall prospect throws in the low-90s and leans on his slider and plus changeup, and that package could work once through a Major League lineup at least right now.

Possibility

Gavin Lux, shortstop, Dodgers: This is a dream for Dodgers fans and prospect-philes alike. Lux has been on fire since joining Triple-A Oklahoma City on July 6, hitting .396/.480/.734 with 13 homers in 47 games entering Thursday. Only Padres infielder Ty France (195) has a wRC+ higher than Lux's 191 among Triple-A batters with at least 200 plate appearances at the level. There is little else for the No. 10 overall prospect to prove offensively. Additionally, he has started to get more looks at second base, where he can get out of Corey Seager's shadow in Los Angeles. The Dodgers themselves left the door open for Lux this week with manager Dave Roberts telling reporters that the infielder is "in the conversation" for a move in September, though he added that Lux could also be around the club without being on the roster. There is no doubt the left-handed slugger could help the NL West leaders in some capacity, much like former teammate Dustin May has. It just may come down to whether the Dodgers are willing to start Lux's service-time clock.
Jesus Luzardo, left-handed starter, Athletics: It's been a bit of race for Luzardo to show both health and Major League readiness over the last month. After hoping to contend for a rotation spot in Spring Training, the 21-year-old left-hander has been limited by shoulder and lat strains this season but is building up his strength at Triple-A Las Vegas. He last pitched Sunday, throwing 84 pitches over 4 2/3 scoreless innings. That was his longest outing of the season in terms of both innings and pitches. He's stretching out while still hitting the upper-90s with his fastball -- both positive signs for his chances to crack Oakland at some point. The A's need all the help they can get in their Wild Card hunt -- it's why they brought up A.J. Puk -- and there's no doubt Luzardo's fastball-curve-changeup mix could be of service right away. He just needs to stay healthy to make it happen.
Ian Anderson, right-handed pitcher, Braves: There were good reasons to put Anderson in the "Don't hold your breath" category. He just came up to Triple-A Gwinnett on Aug. 5. He has been so-so in four starts at the Minors' highest level, posting a 5.48 ERA and 1.41 WHIP in 21 1/3 innings. His 132 1/3 frames on the season are already a career high. But it's not completely out of the question the Braves could bring up their top pitching prospect to eat some innings down the stretch, as they did last year with Wright, Bryse Wilson, Touki Toussaint and others. General manager Alex Anthopolous didn't rule it out, telling The Atlanta Journal-Constitution earlier this month that the organization will go with "[w]hoever gives us the best chance to win." That could be the No. 32 overall prospect, who has three above-average pitches in his fastball, curveball and changeup. The Braves' time to compete is now, and that could mean it's Anderson's time, too.
Brusdar Graterol, right-handed pitcher, Twins: The move wasn't explicitly about getting him ready for a callup, but that may have well have been its purpose. Graterol missed roughly two months with a shoulder injury, and when he returned to Double-A Pensacola on Aug. 8, he did so exclusively in a relief role. After opening with five scoreless innings spread across three outings and 11 days, he moved up to Triple-A Rochester. So far, the results with the Red Wings have been mixed -- two scoreless outings followed by a three-run, one-inning clunker on Tuesday. The allure of the 21-year-old right-hander working out of the bullpen is obvious; he regularly touches triple digits with his fastball in shorter stints and has a plus slider that is easily his best secondary pitch. And after the earlier injury concerns, it'd be better for him to limit his workload in 2019, anyway. Does that still allow him to join the Twins? It helps that the team is trying to hold off the Indians in the AL Central and that Graterol is Rule 5-eligible this offseason, meaning he'll have to be added to the 40-man soon. Whether the Twins want to do that before the season ends or wait until the November deadline remains to be seen.
Deivi Garcia, right-handed pitcher, Yankees: Similar deal here, although the Yankees have been a little more forthright in saying that if Garcia is to make the big club in 2019, it'll be as a reliever. The 5-foot-9 right-hander broke onto the scene by striking out 120 over 71 1/3 innings between Class A Advanced Tampa and Double-A Trenton. He moved up to Triple-A Scranton/Wilkes-Barre in mid-July and has made his last three appearances out of the bullpen, giving up three earned runs while fanning 14 and walking six in 8 2/3 innings. Stuff-wise, MLB.com's No. 63 overall prospect could probably handle a few batters at a time with his plus fastball and plus-plus curve. Finding enough innings in an already loaded bullpen could get in the way of a potential debut. Then again, Garcia is as good as they get in the Yankees system, and his skill set might be too much to ignore for a club trying to secure yet another World Series title.
Ryan Mountcastle, first baseman/outfielder, Orioles: The down-trodden O's have to add their No. 4 prospect to the 40-man roster this offseason and might want to get a closer look at him before then. Mountcastle certainly did plenty to make that an attractive possibility. He was named the International League MVP this week and sits with a .309/.340/.531 line and career-high 25 homers over 123 games with Triple-A Norfolk. The bigger questions surround his defense, and manager Brandon Hyde seemed to indicate that the organization is pleased with the way he has progressed on that side of the ball in 2019. Mountcastle's arrival would bring a little excitement to Baltimore at a time when the club would otherwise be playing out the string.
Monte Harrison, outfielder, Marlins: MLB.com's No. 85 overall prospect is already on the 40-man roster and recently returned to Triple-A New Orleans after wrist surgery knocked him out for two months. When healthy, Harrison has loud tools, including a plus-plus arm, impressive speed and good power from the right side. He has also done a better job at making contact in 2019, bringing down his K rate from 36.9 percent last season at Double-A Jacksonville to a more palatable 29.7 percent this summer. Harrison hasn't knocked down the door like some others on this list, but he does represent a big piece of Miami's rebuild and that, along with his roster status, could earn him at least a look in September.

Don't hold your breath

Jo Adell, outfielder, Angels: There's no doubt it would be fun to see MLB.com's No. 4 overall prospect play the outfield next to Mike Trout, but it's not likely in the cards this time around. The news that Adell is headed to the Arizona Fall League in the middle of the month was the biggest indicator, but it didn't help his case that he put up a .699 OPS over his first 23 games with Triple-A Salt Lake. There's every reason to believe that Adell, who missed serious time with hamstring and ankle injuries suffered in Spring Training, will debut in 2020, putting true five-tool potential next to one of the best five-tool players the game has ever seen.
Luis Robert, outfielder, White Sox: In a just world, Robert might already be up with the White Sox. MLB.com's No. 5 overall prospect has been one of the Minors' best hitters at any level, batting .331/.380/.628 with 31 homers and 36 steals between Class A Advanced, Double-A and Triple-A. He played 43 games for Charlotte entering Thursday, too, so his sample at the higher level (where he has a .993 OPS) isn't small by any means, either. And yet with the White Sox out of contention, the club isn't likely to start Robert's service-time clock over some meaningless games in September. He has a better chance to crack the 2020 Opening Day roster a la Pete Alonso, Fernando Tatis Jr. and Chris Paddack.
Dylan Carlson, outfielder, Cardinals: The Cardinals already explicitly ruled out bringing up the Texas League MVP, and his move to join Adell in the AFL backs that up. Still, the 20-year-old switch-hitter could make that decision tougher and tougher if he can build on his .377/.431/.642 line through his first 14 games with Triple-A Memphis.