Four prospects who got an early start
Some teams didn't want to wait for big league rosters to expand from 25 to 40 in September and called phenoms up in August:
Taijuan Walker, RHP, Mariners (No. 4 on MLB.com Top 100): Seattle isn't playing for anything but an early pick in the 2014 Draft, yet it decided to give MLB.com's top-rated pitching prospect a look in anticipation that he'll crack next year's rotation. Just 21, Walker stands out for his athleticism and his mid-90s fastball. His cutter and his curveball have their moments, though he's still developing his command. Walker beat the Astros in his first big league start, allowing only an unearned run in five innings on Friday. Because he has worked a career-high 146 1/3 innings, the Mariners will shut him down before season's end.
Xander Bogaerts, SS/3B, Red Sox (No. 6 on MLB.com Top 100): Jumping into the middle of a pennant race at age 20 hasn't fazed Bogaerts, who has gone 6-for-19 (.316) in his first two weeks with Boston. He has the potential of doing for the Red Sox what Manny Machado did for the Orioles in 2012, though Boston has eased Bogaerts into the Majors. He's spelling Stephen Drew at shortstop (his natural position) versus left-handers and filling in for Will Middlebrooks at third base (which he hadn't played before this year). Bogaerts will have 30-homer power in his prime, and he's also capable of hitting for a high average and staying at shortstop.
Travis d'Arnaud, C, Mets (No. 21 on MLB.com Top 100): When the Mariners promoted Mike Zunino to the Majors in June, it left no doubt that d'Arnaud was the best catching prospect in the Minors. His solid offensive and defensive skills had made him a key part of trades for Roy Halladay and R.A. Dickey. The only real knock on d'Arnaud is that he's had a difficult time staying healthy, and he missed three months this summer after breaking a bone in his left foot. New York traded John Buck shortly after promoting d'Arnaud, and though the 24-year-old is off to a 7-for-40 (.175) start with one homer, the important thing is getting him ready for a starting job next year.
Sonny Gray, RHP, Athletics (No. 3 on A's Top 20): Gray was the best pitcher in the Triple-A Pacific Coast League for the first four months of the season and Oakland's second-best starter in August, going 2-2, with a 2.90 ERA and 31 strikeouts in 31 innings to net four quality starts in five tries. The 23-year-old has a quality fastball-curveball combination and strikes a nice balance between locating his pitches and being effectively wild. He could make a significant impact in the AL West race.
Four prospects just called up
Clubs quickly summoned these prospects once rosters increased to 40 on Sunday and the Minor League season ended Monday:
Nick Castellanos, OF, Tigers (No. 11 on MLB.com Top 100): One of the best pure hitters in the Minors, Castellanos made significant strides with his power and discipline this year, setting career highs for homers (18) and walks (54) as a 21-year-old in Triple-A. He did so while becoming a full-time outfielder for the first time in his four years as a pro. Castellanos has more offensive upside than the Andy Dirks /Matt Tuiasosopo platoon Detroit has used in left field, though it remains to be seen how much the Tigers will play Castellanos as they try to secure home-field advantage throughout the AL playoffs.
Michael Wacha, RHP, Cardinals (No. 16 on MLB.com Top 100): It was a surprise when a college pitcher as polished as Wacha lasted 19 picks in the 2012 Draft. He has made several teams regret that decision by dominating in the Minors and making his Major League debut in May, less than a year after getting drafted. Wacha spent most of August in the St. Louis bullpen, striking out 23 in 15 2/3 innings, before getting sent down temporarily to throw a side session in Double-A and return as the Cardinals' No. 5 starter. The 22-year-old Wacha has the fastball, changeup, command and poise to pitch well in September -- and in the playoffs.
Billy Hamilton, OF, Reds (No. 17 on MLB.com Top 100): No player is as quick on the bases as Hamilton, who stole a Minor League-record 155 bags in 2012 and finished second in the Minors with 75 swipes this year. He also made a smooth transition from shortstop to center field. Hamilton, 22, is still raw at the plate, so Cincinnati won't trust him with many starts as it fights for the National League Central title. But the Reds should use him liberally as a pinch-runner and a defensive replacement, and he could make a difference in the late innings of close games.
Erik Johnson, RHP, White Sox (No. 3 on White Sox Top 20): Coming into the year, Johnson was a relatively unheralded 2011 second-round pick who had missed the first part of 2012 with shoulder fatigue. He since has established himself as the best pitching prospect in the White Sox system, a potential No. 3 starter with the makings of two plus pitches in his fastball and slider. Johnson, 23, missed much of July with a strained groin but has been untouchable since. That injury also kept his innings down, giving him the opportunity for some September exposure.
Two prospects we'd love to see but probably won't
Their teams are out of contention, and there's no need to add them to 40-man rosters this offseason, so these two guys probably won't see the Majors this September. But we can dream:
Archie Bradley, RHP, D-backs (No. 7 on MLB.com Top 100): Walker's closest challenger as baseball's best pitching prospect, Bradley might be the University of Oklahoma's starting quarterback right now if he hadn't signed for $5 million as the seventh overall pick in the 2011 Draft. He jumped from Class A Advanced to Double-A in May at age 20 and finished the year third in the Minors in wins (14) and ERA (1.84) and sixth in strikeouts (162 in 152 innings). Bradley can overmatch hitters with both his mid-90s fastball and his hard curveball, and he made strides with his control and command in 2013.
George Springer, OF, Astros (No. 24 on MLB.com Top 100): Springer put up the best numbers in the Minors this year, batting .303/.411/.600 with 37 homers, 108 RBIs and 45 steals. He recorded the Minors' first 30-30 season since Grant Desme in 2009 and nearly became the first 40-40 performer since the modern era of the Minors began in 1962. The 23-year-old Springer's power and speed are for real, and his center-field skills and his arm are plus tools as well. Though he can too aggressive and miss offspeed pitches at times, he'd be Houston's biggest offensive threat right now if given a chance.