Ronaldo was named on the bench but left his seat to return to the dressing room before the final whistle despite manager Erik ten Hag still having two available substitutions.
Ten Hag refused to expand on the situation during his post-match media duties, saying only that he would address the issue on Thursday.
"I don't pay attention [to that] today, we will deal with that tomorrow," Ten Hag said when asked about Ronaldo's behaviour in leaving the bench in the 90th minute.
"The focus today was on a magnificent performance from all the 11 players.
"I have to correct myself, not just 11 players but the substitutes who came on, it was a squad performance.
"We deal with that [Ronaldo] tomorrow. What we see today is 11 players who defend and 11 who attack."
It's not the first time Ronaldo has caused a problem for Ten Hag by leaving a game early.
The Dutchman branded the striker's actions "unacceptable" after he was pictured leaving Old Trafford before the end of a preseason friendly against Rayo Vallecano in July following his substitution at half-time.
"This is unacceptable," Ten Hag said at the time. "For everyone. We are a team and you have to stay to the end."
Ronaldo, who has scored two goals in 12 appearances so far this season, was replaced with 18 minutes left despite United needing a goal, but Ten Hag insisted he did not have a problem with the reaction.
"I think no player is happy when he [is taken] off, and especially not Ronaldo, I understand that," he said.
"As long as it is [done] in quite a normal way, [I have] no problem with that."