Pelfrey’s Splitter

Last night, Mike Pelfrey picked up his fifth win of the season, throwing seven and two thirds innings of two-run ball against Atlanta. Although Pelfrey is not as good as his traditional numbers suggest — those five wins and a 3.02 ERA — he has pitched slightly better this year than previously. His xFIP is 4.09 this year compared to a 4.60 career average. That change is largely the result of his increased strikeout rate: 6 K/9 this year, almost a full strikeout per nine more than his career average.

That could just be noise, but Pelfrey is doing things much differently this year. Previously he threw almost 80% fastballs (relying heavily on his sinker) and 13% sliders along with the occasional curve or change. This season he has dropped his fastball percentage down below 70% and has started throwing a splitter, doing so 17% of the time.

This shift is seen dramatically when he gets to two strikes. Before this year he still threw lots of fastballs in these counts, 76% of the time. Fastballs have, on average, the lowest whiff rate, so going with a fastball in a two-strike count is not the best way to get a strikeout. As a result most pitchers throw fewer fastballs in two-strike counts and instead go with breaking or off-speed pitches. Pelfrey’s two-strike fastball rate was quite high compared to average. This year in two-strike counts he has thrown just 62% fastballs, and goes with his new splitter 24% of the time. I think these two-strike splitters, which should lead to more whiffs than his fastballs, play a big role in his increased strikeout rate.

These splitters could also help Pelfrey against LHBs (against whom his career xFIP is an ugly 5.22). Sliders, previously Pelfrey’s main non-fastball pitch, generally have a large platoon split and are ineffective against opposite-handed batters. Splitters, like changeups, have little to no platoon splits, so the pitch could be an important tool for Pelfrey against LHBs.

At this point it is too soon to say that Pelfrey’s splitter will lead to an increased strikeout rate, smaller platoon split or, generally, make him a better pitcher. But the early indications are promising.