In light of this new revelation, I went back and checked some old chapters that might be relevant.
1) We don't see Grand Fisher kill Masaki
If it had been Grand Fisher, why didn't it eat both her and Ichigo? We're to believe it killed her, left her body, left Ichigo, and then left the scene? At the very least someone had to have interfered to make him retreat. Unlikely to have been Isshin based on his later confrontation with Grand Fisher. I guess it COULD have been Masaki herself, (We have no way of knowing her strength level) but the way she seemed to be shielding Ichigo suggests to me a last resort that still requires the presence of some outside assistance to actually defeat or deter the attacker. Unless, of course, the attacker was not a Hollow..
2) Grand Fisher didn't remember Masaki or Ichigo
It was only able to recreate Masaki's form after extracting the memory from Ichigo. This may or may not be important.
3) Isshin regrets not being able to save her - at a different time?
The translators notes here make me wonder if they were wrong to second guess Kubo like that. Perhaps there's a good reason for this seemingly inconsistent line.
http://www.mangahere.com/manga/bleach/v22/c188/9.html