If Iaquinta is not available then Del Piero definitely should start, I see some people who takes this even further, they put formations with Palladino as a starter, that's what I call unacceptable! But If both Iaquinta and Trezeguet are fit, they should be in the starting eleven. any of them under-perform, then Del Piero comes in and he will definitely will give more than he would as a starter.. Not to forget he won't play forever, we need to find replacements, it's already late that we didn't do that till now..
I will criticize the board here for not bringing a young forward for the future, neither Iaquinta (because of his age) nor Palladino (because of his limited skills) would take his (starting) place. Moggi's Idea back then was a good job by bringing Zlatan, we have two aged fstarting forwards, and the young talents out there are already bought by other major clubs, we better do this before the season ends..