It's actually quite simple. Even if Bob Watson cuts down Melky's suspension it'll likely only drop from 3 games to 2 games. For 2 games the Yanks will have no one to backup Damon in centerfield.
The current plan is to carry 13 position players (Posada, Molina, Giambi, Duncan, Ensberg, Cano, Arod, Jeter, Betemit, Matsui, Damon, Melkly, Abreu).
The Yanks need to either start one of the borderline pitchers in the minors to clear a spot for Gardner, or start Duncan in the minors.
Either way, as soon as Melky's suspension ends you send Gardner back down so he can get regular ABs in Scranton. Personally, I'd rather Shelley start in the minors so that he and Melky serve their suspensions consecutively rather than concurrently. It's better to play down one player for 4 games than down 2 players for two.