User blog comment:Xilinoc/Ch. 581 - THE HERO 2/@comment-24153155-20140521225952/@comment-7079377-20140522004714

What I think Shinji meant was that Kenpachi is a big aspect in the war for the Shinigamis. In terms of fighting against Ywach or Haschwalth, and not that anyone can not fight/take on the other Sternritter. As Shinji seemed to believe that he can take on Bazz-B, when he reached for his sword and egged Bazz-B on, asking "Who are you? Wanna get in the way?" (My favourite part of this strong chapter). http://www.mangareader.net/bleach/581/8

As for Byakuya being there when Rukia released and used her Bankai for the first time, I think he was out of the attack radius as you mentioned, and he could have just used shunpo to get to her side and help.

Are there not 2 War Potentials that we do not know of? We know Ichigo, Aizen and Kenpachi. So for all we know they are on the field.