Dan Roche just tweeted the news out a few minutes ago that Alfredo Aceves will be the closer. It's a little bit surprising that Melancon isn't getting the spot since he has closing exprience but given how he fared in the AL East when he was with the Yankees (4.87 ERA in 15 career appearences), they may want to try and ease him in as the set-up guy.
This also hurts the bullpen as they lose out on Aceves' versatility. This means that Andrew Miller will be used as the spot starter and that's a tad bit shaky. When he's good, he'll give you 6 innings of 2 run ball, but when he's bad.. oy vey.
Aceves will do well but he's not going to dominate and be like good ol' Papelbon. This will be the biggest challenge in his young career. If it's anything like he's shown us last season, it'll be a nice surprise and calm all of our nerves down.