Several reasons for this:
1. They needed another big body for special teams due to Gronkowski's ankle. I doubt you see Gronk on teams at all.
2. Belichick has always carried extra d-linemen in indoor and warm weather games due to the fatigue factor. With Carter out, they felt they needed an extra rusher.
3. Underwood only saw two snaps in the AFCCG. Not a huge loss to the offense. Two more snaps for Ochocinco to run down the field and draw some coverage. No real difference there.
4. Underwood probably knew about this for a few days. He still gets to be in the team hotel, hell he'll probably be on the team bus over to the stadium and in the box with injured players/practice squad guys. He'll get a share, a ring if they win, etc...
5. They waited until tonight so the Giants could not sign him in advance of the game.