Corey Knebel, Milwaukee Brewers
Entering the 2017 season, Corey Knebel never posted a strikeout rate higher than 28.2% or an ERA lower than 3.22 in a single season. In fact, he produced just 0.7 fWAR over his first 91.2 big-league innings. He exited the 2017 season with a 40.8% strikeout rate, a 1.78 ERA, and 2.7 fWAR in 76 innings.
This past year wasn’t necessarily bad, but he regressed rather heavily when compared to that breakout performance. In 55.1 frames, Knebel was worth 1.0 fWAR with a 3.58 ERA.
The good part? He kept his strikeout rate high (39.5%) while lowering his walk rate to 9.9% (it was 12.9% in ’17). The bad part(s)? His homers allowed per nine innings spiked (0.71 to 1.14), as did his hard-hit rate allowed (32.6% to 48.6%), while his strand rate (91.9% to 75.7%) and soft-hit rate allowed (21.3% to 11.0%) plummeted.
In just a matter of a year, he went from being an All-Star closer to being one of three options for Milwaukee heading into the upcoming season. The right-hander really needs to get his peripherals at Miller Park under control, too — opposing hitters posted a 22.8% line-drive rate, 43.9% ground-ball rate, 31.6% homer-to-fly-ball rate, and 54.4% hard-hit rate against him at home (19.2%, 51.9%, 6.7%, and 42.3%, respectively, on the road).