I know we already have Wagner and Saito but having Downs would give us an extra reliever that can close out games or pitch later innings leaving Bobby with some freedom as to when he wants to use Moylan for some Double Plays and not be worried about who's pitching the later innings.
This isnt something I believe we should do now, I think we should start the talks and just let them know that we will trade them an Outfielder and a Prospect for Downs if Heyward makes the team. Then I would give them their choice of Melky or Diaz + one of our pitching prospects.
The Bullpen would then stack up as:
Left Medlen off just in case the team decides to have him in AAA as a starter.