He said for this fight he was working on his stamina and he would have the energy to go all 12 with a high output. However again after round 8 he looked badly fatigued. In fact, I thought Canelo seemed more gassed in the late rounds of this fight then in the first fight. Canelo also didnt come in as muscular for this fight as he did for the last fight so the muscles cant be the reason.
It's only a matter of time before someone exploits this against Canelo. This is why GGG needed to attack the body. He might have dropped him late in the fight because body shots tire the opponent out too.