He signed him with Valencia from Auxerre, and with Liverpool, so he does have faith in him. Plus, would you start Sissoko when you have Gerrard, Xabi Alonso and Mascherano?
Obviously it's shit to see Del Piero come off in a big match, especially when he is playing well. But there is just no way to have him on the field when playing with 10 men tactically.
We do need three points. Thank to Sissoko we almost have no chance to get out of this game with one point but we had/have to take risks. Alex could still be our most dangerous player because of his freekicks and also his experience which could have come to help us in such a game.