See my other post on Sox Prospects take on Rafaela. He's much, much better in CF than at SS. Plus don't forget what happened when Kike was moved to SS. We absolutely, positively know Rafaela can plays centerfield because we've freaking seen him do it.
And, by the way, what is your definition of someone in the minors who is ready? If you are talking about a bat, it's always a gamble because MLB pitching is so, so tough. But, if its a glove and an arm, what's great in Worcester or even Portland is probably going to be pretty good in Boston.
Assuming Hamilton is temporary, keep him around until Mayer has some games and shows the good defense ("very fluid actions, soft hands, solid footwork, plenty of range, confident defender, plus defender at SS"), then give him a shot.