So, I sat in a perfect spot to watch Rosenthal pitch Friday night, section H, row 6, right behind home plate and not too low to have my view obstructed. Here is a photo of Rosenthal making top prospect Wil Myers look silly.
Rosenthal is a nicely built kid with broad shoulders and an athletic frame. Just from looking at him, you can see how he could have been a position player with nice athleticism. His delivery is very sound and repeatable, and he doesn't show the ball to the hitter too early, or tip pitches in an obvious way.
From watching Rosenthal it's clear that arm strength is his best attribute. The stadium gun is notoriously fast in Hammons (for instance, it had Mark Mulder throwing 97 on a rehab assignment there once), but evne with a fast gun you could tell that Rosenthal throws hard. He registered 100 on the gun twice, which tells me he was sitting 94-97 with his four seamer. He likes to use it up in the zone, and while it has great velocity, it doesn't seem to be a pitch that is unhittable. Batters routinely fouled off well placed four seamers to wait on a better offering to hit. Rosenthal did throw a few good curveballs with downward break (it's more a slurvy pitch than a true slider or curveball), but I wouldn't call it even an average major league pitch right now due to inconsistency.
Athletic on the mound, he moves well, has control of his emotions, sems to respond well to giving up base runners, and fields his position well. He's merely average (or a tick below) at holding runners. Didn't see him throw over much, which is usually an indication of a pedestrian move.
The entire time I'm watching Rosie pitch, I kept thinking, "this kid should not be giving up so much hard contact with this stuff." He struggled throwing strikes, as the line will tell you, but a lot of his problem was an inability to put away hitters. Hitters would continuously get a piece of pitches to stay alive and eventually he'd lose the zone. I thought the strike zone was fine, and did not play into his line.
His velocity is good, but it's fairly straight. When he goes to the two seamer and gets ground balls, he had better results in my opinion. He needs continued development of his secondary pitches to remain a solid big league starter. If he relies solely on velocity in short stretches, and doesn't add consistency to his breaking stuff, he will have to transition to the bullpen in my view.
Young for the level, we're a long ways away from having to make that move, but at the moment, he's along way from the big leagues. Impressive arm, solid prospect, this outing did nothing to raise or lower his stock in my mind. He's a great arm with a #3 ceiling, and a 7th inning floor.