User blog comment:Eduardovongola/Renji Overpower/@comment-3219912-20131229073231/@comment-3219912-20131230073340

I feel as though "a few days of training" isn't really a valid argument in Bleach. Ichigo and Renji were both able to learn Bankai and rival Byakuya in just three days with Yoruichi (though Renji just managed to force Byakuya to use his Bankai which Byakuya admitted to be impressive). So I think it is plausible for Renji to become super strong in a short period of time while training the Royal Guard, who are stronger than Gotei 13.