Each week, BA will take a look at the trends in major league baseball.
Matt Olson, 1B, Athletics
Olson has played exactly 54 games in the major leagues this year. That's been enough for him to rank tied for sixth among all rookies in home runs already. The 2012 supplemental first-rounder has been on a tear of historic proportions the last few weeks. Olson has 14 home runs in his last 20 games, including a homer in four straight entering today. His 22 home runs overall on the year are tied with Paul DeJong (95 games played) and Ian Happ (104 games played) on the rookie leaderboard. Olson's performance hasn't been cheap, either. His 93.3 mph average exit velocity, per Statcast, is fifth in the majors behind only Aaron Judge, Miguel Sano, Nelson Cruz, and Joey Gallo.
Luis Castillo, RHP
After years of their top pitching prospects getting blown up as soon as they reach the majors, the Reds may have finally found a keeper in Castillo. The 24-year-old righthander, acquired from the Marlins in the Dan Straily trade last offseason, leads all rookie starters with a 3.12 ERA in 15 starts. He is averaging nearly 10 K/9, is keeping his walks reasonable (3.2 BB/9), and has been one of the hardest pitchers to hit, with just 6.4 hits-per-nine innings allowed and a .202 opponent average. Castillo's arsenal is headlined by a 97-101 mph fastball, but it's been his 87-90 mph changeup (.124 opponent average) and 85-88 mph slider (.071 opponent average) that have given hitters the most trouble.
Nicholas Castellanos, 3B, Tigers
The Tigers are building for the future, and Castellanos is delivering a strong finishing kick in his case to be a part of it. After struggling for most of the year—he was hitting .236/.303/.429 on Aug. 8—Castellanos has hit .371 with 13 doubles, seven home runs, 34 RBI and a 1.024 OPS in his last 35 games. That includes a current 15-game hit streak, during which he has nine multi-hit games.
Guillermo Heredia, OF, Mariners
The Mariners are desperately trying to hang on in the playoff race, but their rookie starting center fielder hasn't given them much help. Heredia is last among all qualified American League rookies with a .672 OPS, and he's been getting worse as his team needs him most. Since Sept. 1 Heredia is hitting .157/.232/.136—a .409 OPS that is worst in the majors in that time.
Dan Straily, RHP, Marlins
While Castillo is flourishing in Cincinnati, Straily is scuffling in Miami. After performing well most of the year, Straily has gotten lit up for a 6.86 ERA and ML-worst 1.95 WHIP in September. That includes opponents batting .360 off of him with five home runs in 21 innings. Straily's struggles are part of the reason why Miami has seen it's slim playoff hopes disappear. They entered Sept. 2 five games out of a playoff spot, but since then have 3-12, the worst record in the National League.
Chris Davis, 1B, Orioles
"Crush" is another player who got cold at the wrong time. With the Orioles trying to make a postseason charge, Davis his .145 with 25 strikeouts in 63 plate appearances this month, the latest tough stretch in what's been a trying season. Davis, 31, is on pace for his fewest home runs and RBIs in a season since he came to Baltimore in 2011, and his .217/.312/.428 slash line marks at least a 40-point drop in every category from two years ago.