With the match's final score of 1-1, Ronaldo was taken off and replaced by Andre Silva in the 65th minute.
South Korea advanced to the round of 16 together with Portugal thanks to a late goal from Hwang Hee-chan that handed them a 2-1 victory.
Despite Ronaldo's exhortations for his teammates to learn from their terrible performance, Portugal was unable to salvage anything from the game after he had long since left the field.
As he left the field, he described his feeling as follows: "Prior to departing, [a] Korean player instructed me to go right away. I told him to stop talking.
"He is not the decision-maker, he is not required to provide an opinion, and if he were the referee, he would expedite the process. There needn't be any contention, though.
"The game is becoming heated up. Whatever transpired, everything remains on the field. We must work together since the next stage has begun. We must all have confidence in Portugal if we are to advance.
"We were aware that we were eligible, so it would have virtually taken a miracle for us to advance [in] first. But I don't want to use that as an excuse.
"When we don't play well, we must draw lessons from these experiences. We need to win the following game, and I hope we can learn from this setback."
Regarding Portugal's performance, Santos added: "This is disappointing. We first did good deeds, but we later switched to doing bad deeds. But it might also be a caution. It might be an alert."
Next Tuesday, Portugal will play the Group G runners-up in the round of 16.