What's Up With St. Louis Cardinals' Trevor Rosenthal?

The St. Louis Cardinals are having major bullpen issues in the wake of Jason Motte's potential season-ending elbow injury. Mitchell Boggs seems to be a headcase in the ninth inning with his horrendous ERA and a nearly one-to-one BB/K rate. One-time closer Fernando Salas and Mark Rzepczynski are having a tough time as well and don't seem suited for the late innings.

The man who might be best suited for the job out of the remaining 'contenders' is fireballer Trevor Rosenthal. Yet how can Mike Matheny and co. feel comfortable running out a young man who has already blown two saves in eight games through nine innings pitched?

Well, for one, its a really small sample size—albeit a very disconcerting one. Rosenthal currently has a .400 BABIP and is leaving just over 60% of runners on base.

This could be the result of some shoddy defense—which the Cardinals are not above from time to time. His FIP is over three runs lower than his ERA and he has a stunning 11-to-1 K/BB rate. Rosenthal looks to be holding his end of the bargain up as a pitcher, right?

Maybe not.

Rosenthal's line drive and ground ball rate are well above the Major League Baseball average at 25 and 50 percent respectively. For a guy with a lights-out fastball that tops out at nearly 100 MPH, this is pretty odd—especially with a 14% swinging strike rate.

That being said, the problem I'm seeing with Rosenthal is he's not varying up his pitches. Right now he's throwing his fastball on over 80% of his pitches. The rest—a slider, curve and change—are all being thrown less than 7% of the time. So it's no surprise what hitters are expecting when they come to the plate. That also explains his high HR/FB rate of 16.7% (25% FB rate).

I would be very surprised if Rosenthal maintains that high BABIP—and HR/FB rate—for an extended period of time. I'll be even more surprised if Rosenthal doesn't start mixing his pitches better. He could simply be having a bout of bad luck but I seriously doubt that. If you know four out of every five pitches is going to be a fastball, how hard will it be—for even the most average hitter—to time it and drive it?

The curve ball is going to be key for Rosenthal to succeed in the ninth. He simply cannot live and die on his fastball without having a secondary pitch he throws more than 7% of the time. Even his fellow division flame-thrower Aroldis Chapman mixes in his slider on about 11% of his pitches.

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