If you wanted to argue that Wily Peralta has been the Brewers best starting pitcher, I wouldn't really argue with you. I'm not sure you'd be right, but he has the lowest ERA in the rotation so that's pretty cool. He definitely seems to have improved on last year. It's a little too early to say this is the pitcher he is, but I have noticed something to keep an eye on going forward. This year he is absolutely brutalizing left-handed hitters.
Peralta is holding lefties to a 203/247/273 slash line and has a 2.79 FIP against them. He has been striking them out more this year, walking them way less, and inducing more ground balls. The really striking difference, however, is in infield fly-balls. He's getting lefties to pop-up to the infield 22.2% of the time. That's insanely high.
Brooks Baseball tells us that he's attacking both sided hitters low and away. With two strikes against RHH, Peralta has used a pretty equal mix of his four seamer, sinker, and slider. With two strikes against LHH, Peralta has relied heavily on his slider. It's his put away pitch to lefties, but he's also getting them to pop-up with it a whopping 20% of the time they put it into play.
Peralta's overall 16.1 IFFB% is double what it was in 2013. Last year's MLB leader had a 15.6 IFFB% so Peralta's is due for regression sooner or later. That means his numbers against lefties are very likely to see some regression too. He still has a better K%, BB%, and GB% vs LHH this year. That's encouraging in the early going and it's something to keep an eye on for the rest of the season. LHH was an issue for him last year so figuring out how to attack them would go a long way towards him taking that "next step."
Statistics courtesy of Fangraphs
Pitch usage and outcomes courtesy of Brooks Baseball