I doubt its someone in Aizen's crew. Stark is currently taking down two vizards at once, so why would Aizen stop that from happening?
It could be Urahara, but to be honest I think hes going to end up going into Hueco Mundo to fight Yammy. Ichigo and gang needs a way back to the real world, and Kisuke can open up gargantas. Kisuke also has run into Yammy a few times so they have a score to settle. Given the fact that he and Yammy have history, and he knows how to open a garganta to get Ichigo back to Karakura town, I think this is the most likely thing for him to do.
Here is my official prediction on the mystery swordsman, or should I say swordswoman? Lisa! Stark hurt Shunsui, and she <3 Shunsui, theres no doubt about it. Lisa is the one who did it.