Listen, Manninger did very well. If we had any other 2nd keeper we might have been 7 or 8 pts behind Inter instead of 3pts. But if we had Buffon instead of Manninger we might have been leading the standings right now. So, there is a difference.
I already told you why the importance of Trez would've been big. No need to mention Camoranesi.
We might have squad depth, but sometimes you have only one available player for a certain position and even if he plays awful Ranieri can't replace him simply because we have no other option on the bench. Sometimes, like today, we didn't even have a single player to play in one particular position, because even our boy for every position, Salihamidzic, is injured for a long time already.
I can't believe you downplay the importance of these injuries. As Juve Red told you, imagine Inter without those players and without enough cover on the bench. With time, maybe they'd have found the chemistry and started winning, but for sure there would've been time when they would have lost points due to those injuries. Just look at them with 4-3-3 in CL, even at the start of serie A. It's not simple to change the team every next match, or to change the team so drastically as we were forced to do on many matches.