Board Thread:Questions and Answers/@comment-27127950-20151027213016/@comment-27392440-20160109142653

I would probably agree with you if Aizen hadn't felt the need to get Urahara's hogyoku as well but considering he felt his wasn't complete plus the fact that we were shown that an incomplete hogyoku is unstable (the vizards were going to soul implode and had no control) makes me believe that he probably didn't rely on his own in upgrading powerful hollows to arrancar and just probably relied on the safer but weaker method.