Interestingly, a long line of Yankee stars came out of the Red Sox organization - Babe Ruth, Herb Pennock, Carl Mays, Sparky Lyle, Wade Boggs, Johnny Damon, Jacoby Ellsbury, Luis Tiant, Roger Clemens, etc.
For some reason, the reverse is simply not true, and former Yankees coming to Boston are always flops, with the one exception I can think of as Don Baylor. And he only played in Boston for two seasons. The only other positive contribution from a former Yankee I can think of was from Tony Armas Jr. whose primary skill seemed to be injuring his pitching arm. But at least he was enticement enough for the Expos to trade Pedro. And Mike Torrez was not horrible, I guess...