Fabregas doesn't strike me as the type of person who would join Utd behind Arsenals back. Van Persie is a different personality he's more career driven, focused on success by any means and not the type of player to get emotionally attached to a club. Even when he was at his boyhood club Feyenoord he was pushing for a move to PSV back in 2003 until we came along.
Fabregas seems like an emotionally driven character. I think his desire to join Barcelona was fueled largely by his good childhood friends all winning and achieving things together. Even after he's left Arsenal, he still talks passionately about the club and especially Wenger's influence on his career. I can't even comprehend him joining Utd.