I think you're undervaluing how important Quagliarella was to us in that period. I was watching the game at work and as soon as I saw him go down that way I nearly had an epileptic fit and said to my colleague 'that's our season fucked', that was how important I felt he was at the time. Of course that's because in comparison to the days we had Zidane we are mediocre, but I think it's clear that as a team and our players as individuals we suffered from that. However it doesn't explain what followed as we also had a hot Matri come in and do well, that you can blame on Delneri and other areas such as our coward mentality.
I'm not saying he wasn't good or important, he scored a good amount of goals. But we showed signs of trouble before he went down, from dropping points against Chievo, Brescia, Roma and Fiorentina at home, along with the debacles in Europe. When watching him play, I was most impressed with how Quag would be able to find the right areas in front of goal and capitalize on chances. People say he offered a lot of movement; I wouldn't say that, but Serie A defenses know he can hit long range shots so they had to close him down, maybe opening up space for others from time to time.
Personally, I think the big loss at home to Parma was more traumatizing than losing Quag. It's the first match of the new year, you want to start the year off right, you lose both Quag and then Melo to red, and then you have a former player like Seba tear you apart in front of your own audience. Then in the following match, you have several defensive screw ups and Cavani scores a hattrick. You couldn't have a worse start to the year than that.
So it's not just the loss of Quag that effectively ended our season. It was a conglomeration of stuff, including being incredibly disorganized at the back, that set us off. I have never seen a side as disorganized defensively as we were maintain the same level of results throughout a season anyway. The collapse was coming.