I thought Unohana was clear on why he became weak. She gave Kenpachi a high that he's been unable to find again kinda like a addict trying to reach the same high as the first time. He use's the eye patch to that extent along with the the bells and not going all out at the start. Kenpachi has been holding back for so long that now he is presented with a chance to achieve that first high again, he wants to enjoy it again so he is still holding back even if he isn't concisely doing it. What Unohana is trying to do is correct this.