I don't think Shields was going to move the needle a ton on his trade value in the next 7 weeks, and for the possibility of getting better prospects, there's also the possibility that things get worse for Shields or he gets hurt. I would guess he's thinking that the risk of losing what's on the table (Erik Johnson and Fernando Tatis jr) is not devastating loss if you wait a few weeks and he gets injured or they don't get as good of an offer. My guess is that the trade was financially motivated. Even though they ate half the deal, that's still a $28m savings for a team that's going nowhere. That is what you risk by waiting until late July when the Sox are offering it now.