Guardiola accused Onana of timewasting throughout City's 3-1 win.
He said: "I don't know if he was injured or had problems. It is up to the referee to make that decision.
Advertisement
Advertisement
"There are rules for that. We can set up many things as a team, we want to play and play and play. Then there are teams that bide their time, bide their time and bide their time.
"Then six, seven or eight minutes are added when they don't want to play. I still don't understand the rule, but there is a rule and we have to accept it."