User blog comment:TheBody84/Ichigos father being the former Kenpachi/@comment-1624279-20091025005447

@ Bk1217, 9 & 5 are not likely at all. We saw Tosen use Bankai against Kensei. Also, using his hypnosis Aizen could just show the captains his fake Bankai. With both of these facts, we can assume both Tosen & Aizen immediately took the position of squad captain.

3 albeit more possible, is also unlikely. I don't know if squad 3 just did what its doing now, i.e waiting for someone to become a captain. I assume it did fine with the lieutenant leading until Gin became captain.

7 & 10 are the most likely ones. We don't know how long Sajin has been captain, though i'm inclined to think he may have been promoted shortly after Gin & Byakuya. Toshiro seems to be the newest & youngest of all the captains.

So my money would be on squad 10.