He's not always injured, that shoulder injury is just an unfortunate recurrence of something that happened before. We need a player like Trezeguet to be our point-man, someone we can depend on to finish whether the ball comes in on the ground or in the air.
If we only had creative players like Del Piero and Miccoli playing all the time, we'd score a grand total of 0 goals from crosses all season. I'd only replace Trezeguet if we already purchased a suitable replacement, and slowly phased him in, much like we did with Inzaghi when Trezeguet first arrived.