When he's been playing hurt in the past, his power fades. For whatever reason, at the end of April he decided he didn't like taking walks any more and was going to take a more aggressive approach. I think he's dialed that back some recently, but is still chasing some bad pitches, and has got himself out of sorts and kind of lost at the plate.
As for trading. First of all, there is no way we trade him unless we're completely out of it, and I don't think that is all that likely. If we are completely out of it, he is probably is really sucking and no one will want him as a rental. Lastly, closers usually have way more bidders than corner outfield bats, so there is no way we are getting very much for him as a rental unless a bunch of contenders all lose their sluggers to injury.