So, from what I make of this signing and from the article naming the bullpen in '05, is that it could mean one of these things for the pitching staff:
Diaz will not be a RP in 2005 for the Sox, he will be the 5th starter in 2005.
Garland will be dealt either after the Sox sign a #3 starter, or traded in a deal for a SP because Diaz will be in the rotation.
Diaz, Adkins, or Cotts will be included in a deal for a better SP.