Mayer's very valuable right now. With the amount of injuries he has had, there is risk he could get a serious injury or that his value decreases as the injuries continue. Also, Sox clearly need pitching and Mayer is a way to get some. He's yet another LHB and not the highest defensive ceiling at SS. He's a solid player, but not projected to be an All Star on an annual basis.
Campbell, I wouldn't trade. He's a RHB and can play all over the field, but they need to figure out where to stick him fulltime (unless they want him to just be a weird UTIL type guy). Campbell has also had offensive results that we haven't seen out of Mayer.