That's a possibility, a solid one, but not a fact. Eto'o was a one man team for Inter, scoring and assisting around 90% of all the goals they scored. He was suspended, we wondered and laughed who's gonna score the goals for Inter now, and they ended up beating Parma 5:2 without Eto'o.
I think that Zlatan's presence is making Milan psychologically stronger. Undoubtedly, he makes them a more potent side and one can't ignore his pure footballing contribution, but it's something else that Zlatan adds that makes Milan stronger.
Felipe Melo said it the best: The defenders in Italy are scared of Zlatan. That fact gives the other Milan players more confidence and freedom.