Rosicky told The Sun: "I remember the Birmingham game and what happened to Eduardo. The lads were winning 2-1 but then in the last minute Gael gave away a really stupid penalty.
"At the same time, he played brilliantly all season. He improved immensely and turned into a great defender as a result. But he messed up that day.
"I don't think something similar would happen to him now, because he's a lot more experienced. But it was definitely a match we should have won by a long way.
"We were leading and we even had another huge chance to score. I think it was Bendtner and Adebayor who were bearing down on goal, but Adebayor shot instead of passing and missed.
"There were an awful lot of other similarly negative things happening. Everything began with Eduardo's injury. Then came Gael's error and that thing with William.
"You could say that was probably where the ground started to crumble beneath our feet."
Gallas' behaviour at St Andrew's raised doubts about his ability to captain Arsenal.
But Rosicky, 27, is backing the French defender to recover from his shame and lead the Gunners.
He added: "It was not an easy situation. On the other hand, it was William's first season as captain and it's not easy to wear the armband in a team like this.
"I think he will have learned. There were things he could have done better - but he knows that better than anyone else.
"He's really immersed in everything and that was something he just got immersed in too deeply.
"But otherwise he's a good leader in my opinion and he will continue to be captain even though there was speculation about that."