His departure wasnt a catalyst, Sasuke left and Naruto took the empty spot Sasuke left on Sakura's place, Sakura only wanted to be noticed by Sasuke which is shown on the bench scene, however he acknowledged her but he left and Sakura felt like she lose her ground and went desperately asked Naruto to bring him back, after Naruto failed and seeing how he was hurt because the fact he failed to bring Sasuke back and also witnessing his devotion towards this goal, she found out another goal to her life which was the help Naruto.
And the proof is that after the POAL she was going to visit Naruto first and then Sasuke which was totally different of the things she has done thoughout on part 1, Naruto become someome important to her because he was at her side when she needed the most (his departure) it wasnt a catalyst because in my opinion if Sasuke didnt left the village Naruto/Sakura would never stay on the way it is now.
She already did give up on Sasuke, and that's why she in a way is not doing stuff to bring Sasuke back, she accepted whatever outcome happens on Sasuke's case, she's worried about Naruto obviously but already accepted the idea of Sasuke dying as an example.
Definition of catalyst: Stimulus to change; someone or something that makes a change happen or brings about an event.
The bolded is pretty much an illustration of the definition of catalyst, so either you're unclear on the definition, or you're just trying to be contrary. Whether the development between Naruto and Sakura would have occurred had Sasuke never left, I can't say, maybe it would have only taken longer to come about, or maybe not. But the fact is, Sasuke leaving brought about a change in Naruto and Sakura's relationship, thus, Sasuke acted unknowingly as a catalyst.