Andrew Bailey is out for an extended period of time due to surgery on the ulnar collateral ligament in his thumb, meaning the Red Sox needed to select a new closer. While there's no word yet on whether or not there's a permanent substitute for Bailey, as far as the here-and-now goes, it's going to be Alfred Aceves closing according to manager Bobby Valentine.
Aceves closing means Mark Melancon remains in the setup role that the team acquired him for. As Melancon might be the club's best traditional (i.e., not 100-plus inning with spot-starting capabilities) reliever, that makes a lot of sense: he won't be relegated to just the ninth inning, and can put out fires in the seventh or eighth as they ignite. By the same token, though, if Aceves is now going to become just a ninth-inning pitcher, then this is an odd use of such a versatile arm, one capable of throwing multiple innings at a time. Valentine also says that when Aceves isn't available, Melancon will close, so there's a hint of closer-by-committee here.
Valentine could also use Aceves for multiple innings at a time still, bringing him in during the eighth when necessary, instead of just saving him for the ninth and a fresh frame. It's just disappointing he won't be as much of a utility pitcher for the Red Sox if he becomes The Closer with Bailey out, but given Valentine is already on the record saying how much 90 innings out of Aceves means to the Red Sox, chances are good he won't be stuck in just the ninth.
You wonder if Aceves as closer also has something to do with his not getting the rotation spot that he pitched well enough to earn this spring. He moves from one valuable role to another, giving the Red Sox high-profile innings, and without cutting into their depth any further to do so. Aaron Cook will be in Pawtucket if the rotation needs him, so Aceves is likely to stay in this role as long as he pitches well. We'll see how it plays out, but there's little reason to think he won't succeed in this role, as he has every other where he needed to throw strikes.