Its a good trade, they get a guy who is one year older and three levels higher. Mazara is still only 24 and has loads of potential, and a change of scenery may be just what he needs to tap into it.
It also is relatively low cost roll of the dice and allows the Sox to take on a bad contract to get some more value. This does not preclude the Sox from doing a Betts/Price trade. Mazara/Eloy/others can rotate through LF and DH as Castellanos and Ozuna are still options as well for RF. If Mazara doesn't hit, you are out Steele Walker who was pretty borderline to be a MLB contributor and you non-tender Mazara next winter. Its a low risk high reward move that in no way stops the Sox from adding through the trade market.