There's an old unwritten rule in sports saying you shouldn't lose your job due to injury. It's pretty brutal for a regular player to suffer an injury, be outplayed for a while by a backup, and then wind up on the bench as a result. It's also usually not the right decision to make, since typically these situations are based on small samples
Still, there are examples of this being the right move. Boston should be particularly aware of that, given how number 12 over in Foxboro(ugh) got his job. Really, when it comes down to it each situation should be judged on its own merits, not on the basis of whether or not it fits tradition. Right now, the Boston Red Sox have such a situation surrounding Andrew Bailey and Joel Hanrahan, and really, it's not even close.
When the Red Sox brought in Joel Hanrahan over the offseason, John Farrell made it abundantly clear that he was their closer. It was a move necessitated by uncertainty more than anything else. The Red Sox had a very exciting young reliever in Junichi Tazawa, but his limited experience made it hard to put too much faith in him. Koji Uehara is the sort of pitcher you can trust, but given his age, probably not a good fit for a closing role which would demand more innings from him. Andrew Bailey was brought in to be the closer last year after filling the role with the Athletics, but was a shadow of his old self in Boston.
Hanrahan, however, was no sure thing himself, however. The Pirates' closer had struggled tremendously in the last month of 2012, as the control issues that had plagued him throughout the year finally started coming around to bite him.
So, when the Red Sox say that Joel Hanrahan has a bad hamstring, and that's the reason for his early-season struggles...well, there are reasons to doubt. Maybe his hamstring is hurting, and maybe it is the underlying cause for all this, but frankly right now it just looks like he's the same pitcher he was last year. Plenty of walks, plenty of homers, plenty of trouble.
Andrew Bailey, on the other hand, has been something special. Yes, we've seen some instances where he's come out overthrowing, letting his pitches get away from him and sail up and outside to lefties. He's not been completely infallible, surrendering a couple of runs to this date. But when he's been on, he's been flat out untouchable. His fastball has that extra bit of oomph behind it, and his secondary offering--it's been called a cutter, but on any given day it looks like a slider or even a slurve--has been an absolute wipeout pitch. That pitch, whatever it actually is, is the reason he has a ridiculous 20 strikeouts in 11 innings of work.
This is all to say that Andrew Bailey right now shows no signs of being a short-term fluke. There's no real reason to believe that Hanrahan is the better pitcher now, or frankly even was the better pitcher when healthy. And even if we were to accept that he's the better option when at 100%, we know what was wrong with Andrew Bailey last year--there's a reason he missed the whole first half of the season. We don't really know what's wrong with Joel Hanrahan going back to last year. Farrell and his staff have some ideas, but so far they haven't really been proven out. At the moment, there's no reason to believe that Joel Hanrahan will even be good when he's back, much less worthy of the closing role. For now, that's got to be Bailey's to lose.