Indians starter Justin Masterson has been the exact same pitcher this season, but has seen drastically different results.
Masterson's low arm angle allows lefties to see the ball much better out of his hand. It's led to significant platoon splits over his career. Left-handers have hit .283/.363/.427 against Masterson over his career.
While he's been able to neutralize lefties early this season, there's nothing in his pitch usage to suggest he has utilized a different strategy. Masterson has used the same three-pitch mix for quite some time now, and lefties always hit him well. There's no reason to expect him to continue dominating hitters at this rate. Teams are well aware of the scouting report against Masterson, and opposing managers will likely try and get as many left-handers into their lineups as possible. It's already obvious, but Fantasy owners should be looking to sell now, because this isn't going to last.