I was going to get into an in-depth analysis of what I think the future holds for each of three pitchers called up in September, but with time constraints I'm just going to tackle Jeffress today. Note that the information I'm presenting is Pitch F/X from the TexasLeaguers.com database, which is does not employ any park corrections, so don't take anything as an absolute fact, but it's certainly interesting information to consider.
Jeremy Jeffress has thrown just his fastball and curve since being promoted to the majors, and there's good reason to believe he could be a very effective reliever throwing just those two pitches. I don't know exactly what the front office has in store (recent comments indicate they haven't given up on him as a starter), and the ideal scenario is Jeffress ending up in the rotation. Much will depend on Jeffress himself, as he has indicated that he prefers to pitch out of the bullpen, but that's an issue for this offseason. Right now I'm only concerned with how good his stuff is.
The average fastball velocity is sitting at 96 right now, putting Jeffress in some rare company. 8 pitchers with 40 or more innings this season have an average fastball velocity that is higher, and only two were regular starting pitchers: Stephen Strasburg and Ubaldo Jimenez. It's much too early to draw any conclusions about how hitters will fare against his pitches by looking at swinging and swinging strike numbers.
Things we can look at, however, relate to how filthy his curveball is. The side view of his pitches, which I cannot link because I can't figure out how to save a screenshot to a Mac and upload it to photobucket, shows a trajectory that can be compared to someone like Ben Sheets, with a big breaking curveball that starts much higher than the fastball and finishes much lower. Jeffress's release points have also been more consistent than most. His curve is not going to be very difficult to pick up for the batter, but the break and change of pace should provide him with an effective complement to his fastball and allow him to be an effective reliever the way he is.
To take the next step, however, he is going to need a third pitch, and possibly a second fastball to keep hitters off balance. I remember him working extensively with a two-seam fastball in 2008 and 2009 but it looks as if he has not used that pitch in the majors as of yet. For an example of what I'm talking about with the need for deception, I looked up the side view of Yovani Gallardo's pitches. I can't link it again for the same reason, and I'm out of time to try to figure it out against this deadline. But I can describe it for you: out of his hand, a slider and fastball look like they're coming in at the same trajectory, only to end up in drastically different spots. Even Sheets worked in his changeup enough to keep hitters with a little bit of a question in their minds. I think the best scenario for Jeffress is starting next year in AAA. The Brewers will need to decide if they want to try him in the starting rotation again, but he should be instructed to focus on a third pitch. He can be a good relief pitcher the way he is right now, but the next step will probably require an addition to the repertoire.