Simon Mercedes, RHP
Mercedes has some impressive stuff, including a fastball that hits the upper 90s and has reached triple digits. He also might end up being a reliever in the long run, though, that remains an unanswered question to this point in the young right-hander's career. He's 22 and in High-A for the first time, and while the Carolina League isn't the final answer for how things are going to go, it is a step up from his previous work, especially when you consider he has all of 70 pro innings behind him.
The Sox used him out of the pen for his first game of the year, but that means little when 3-1/3 innings are attached to it. The thing to watch for is whether his velocity drops and his command suffers as the year drags on, as fatigue has been a concern in the past -- just one more reason a life in the bullpen, even if it's an exciting life, might be the one for Mercedes.
Justin Haley, RHP
Walks were a significant issue for the 2012 draft pick during the first half of last summer, but he turned things around by year's end, issuing 3.4 free passes per nine over his last 10 starts. That's not excellent by any means, but it's exponentially better than his early season wildness, and he still finished with nearly a strikeout per inning on top of it. Haley's career is probably going to be as a reliever, but he's one of those arms the Sox will keep in a rotation (or at least on a starter's workload) for as long as possible in order to get him as much experience as they can.
He didn't issue any walks in his first outing of 2014, but he did allow a homer while striking out four. As he's already 23, the thing to watch with Haley is how quickly he can adapt to High-A hitting, as a promotion to Double-A before the year is out would help out the stock of an admittedly old-for-the-level arm.
Austin Maddox, RHP
Maddox is also bordering on old for the Carolina League, but he at least has the excuse of injury to lean on. His 2013 was horrible, as you could surmise given the career 5.14 ERA for a 2012 third-round selection, but there's still a lot to like about an arm that could end up belonging to a back-end reliever. He only recently became a pitcher full-time, as he was a two-way player in college (and a teammate of fellow Red Sox prospect Brian Johnson at Florida), and while his three-pitch repertoire needs improvement, there are three pitches to work already, at least. Getting his control together and adding some movement to his low-to-mid-90s fastball will be the keys for Maddox: if he can't manage either of those things, it might be another rough season.