Having been on the wrong end of too many Boston-to-New York talent exchanges -- Wade Boggs, Roger Clemens, Johnny Damon, not to mention one G.H. Ruth -- the Red Sox are taking no chances with Carl Crawford.
According to the Chicago Tribune, the Red Sox negotiated a clause in their free-agent deal with Crawford that prevents any team they trade him to, to then turn around and trade him to the Yankees. In other words, if they deal Crawford anywhere, there's an automatic no-trade clause to the Yankees in the contract.
The Sox are reluctant to hand out no-trade clauses to players, and Crawford's contract includes just two teams to whom he can block a deal. (The Tribune didn't specify which ones.) But they added one of their own, to prevent the Yankees from brokering a third-party deal that would enable another team to, in essense, acquire Crawford for them.