With just five games remaining before the All-Star break, the Chicago Cubs are concluding an exceptional first half. After two consecutive 83-win seasons, the team has shattered expectations with a 54-37 record, driven by the league’s third-most productive offense (118 wRC+) and a bullpen that has evolved into a shutdown force. While the acquisition of Kyle Tucker was a significant catalyst, the Cubs’ success stems from widespread improvement throughout the roster.
As the team enters the second half, the front office is actively exploring the market for rotation upgrades to bolster their pennant chase. However, the division race remains tight, with the Milwaukee Brewers only 2.5 games behind and both the St. Louis Cardinals and Cincinnati Reds in close pursuit. To secure the division title and become a legitimate playoff threat, the Cubs must maintain their momentum. While some key contributors appear poised to continue their strong play, others may be due for regression.
Here is an analysis of the sustainability of five key players’ performances.
### Sustainable: Pete Crow-Armstrong
No player has defined the 2025 Cubs more than Pete Crow-Armstrong. The former top prospect has ascended to superstardom, posting a .272/.309/.550 slash line with a 137 wRC+, 23 home runs, and 27 stolen bases. His unexpected power surge, second only to Seiya Suzuki on the team, combined with his elite outfield defense, has made him the most valuable player in the National League by fWAR (4.8) and earned him a starting spot in the All-Star Game.
Crow-Armstrong’s underlying metrics suggest his performance is largely sustainable. His .298 batting average on balls in play (BABIP) is in line with league norms, and his .364 weighted on-base average (wOBA) is closely supported by an expected mark of .354. Despite a high chase rate, he boasts an 85th percentile barrel rate, a .260 expected batting average, and a .530 expected slugging percentage. Furthermore, his 97th percentile sprint speed consistently allows him to take extra bases and beat out infield hits, solidifying the reality of his breakout season.
### Sustainable: Michael Busch
Michael Busch has been instrumental in transforming the Cubs’ lineup into a formidable force. With a .297/.382/.562 slash line and a 166 wRC+, he has improved upon his solid rookie campaign by significantly cutting his strikeout rate to 22.6%. His offensive output ranks among the top five in baseball, and he provides positive defensive value at first base with two defensive runs saved.
A look at his advanced metrics reveals a dramatic year-over-year improvement, with elite numbers in expected stats, hard-hit rate, barrel rate, and launch angle sweet-spot percentage. Although he has been primarily shielded from left-handed pitching, he has shown recent progress, delivering two extra-base hits, including a home run, against southpaw Matthew Liberatore on July 5. Having been given time to develop, Busch is making better swing decisions and hitting with more authority, and with expected batting and slugging percentages of .281 and .576, his monster season shows little sign of slowing down.
### Due for Regression: Chris Flexen
Chris Flexen has been a vital component of the Cubs’ resurgent bullpen. As a minor league signing, he has been a pleasant surprise, posting a remarkable 0.83 ERA across 32.1 innings. His career-low walk rate (2.20 BB/9) has made him a reliable option for high-leverage situations and multi-inning relief appearances.
However, predictive metrics indicate significant regression is likely. Flexen’s 3.25 expected ERA (xERA), 3.57 Fielding Independent Pitching (FIP), and 4.50 expected FIP (xFIP) all point to a correction. His success has been buoyed by an unsustainably low .198 BABIP, a 5.3% home-run-to-fly-ball rate, and an exceptionally high 95.2% left-on-base rate. Compounding these concerns is a career-low strikeout rate of just 4.68 K/9. While his ERA was never expected to remain this low, a return to the mean could cause it to rise sharply.
### Due for Regression: Colin Rea
Following a season-ending injury to Justin Steele, Colin Rea stepped up to stabilize the rotation, and his early success was critical. While his 4.13 ERA in 85 innings has been serviceable, his stellar performance in April now appears to be an outlier as red flags have mounted.
In May and June, Rea posted an ERA over 5.00 and had three starts where he allowed six or more runs. His underlying numbers have deteriorated, with his expected batting average (.290) and expected slugging percentage (.489) ranking in the bottom 10 percent of the league. Although his control is solid (2.22 BB/9), it is paired with a low strikeout rate (6.35 K/9), a 4.81 FIP, and a high average exit velocity allowed of 90.8 mph. The quality of contact he surrenders supports his 5.01 xERA, a figure that is inadequate for a team with playoff aspirations.
### Sustainable: Brad Keller
Among the bullpen’s many success stories, Brad Keller’s is particularly encouraging because his results appear highly sustainable. He arrived at spring training as a transformed pitcher, featuring a 97 mph fastball. He has since secured a key set-up role in manager Craig Counsell’s circle of trust, posting a 2.88 ERA and a 3.11 FIP.
Despite a recent difficult outing against St. Louis, Keller’s profile remains strong. He ranks in the 96th percentile in limiting hard contact, evidenced by his 86 mph average exit velocity and 31% hard-hit rate. He also generates groundballs at an elite 61.1% clip, which complements the Cubs’ strong infield defense. Adjustments to his pitch arsenal, prioritizing his sweeper against right-handers, have paid dividends. His performance aligns closely with his 2.96 xERA, suggesting that this is the best version of Keller to date, and his results are built on a solid foundation.
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