I've seen Redknapp bring on Scott Parker when Spurs needed a goal. The fans went mental, but they didn't understand the tactical move. Parker was effectively used as a last man, to allow everyone else to push forward. But the fans couldn't see past the fact he was a DM and therefore took it as show of tactical ineptitude on behalf of the coach, rather than themselves. Bringing on a DM in those circumstances is actually a very attacking move. I didn't see the game or know the role Rossi played when he came on, but it wouldn't surprise me if the idea was to bring on a DM to cover for counter attacks and let the rest push forward.