Zlatan Ibrahimovic joined the Champions League's 40-plus club on a miserable night for Milan, as the Swede insisted: "We have to do better."
A 1-0 defeat in Porto means the Serie A giants have yet to pick up a point through three rounds of Group B games.
They have never begun a group-stage campaign so poorly, points-wise, and it will now take a remarkable effort to claw themselves back into last-16 contention.
Luis Diaz's second-half strike gave Porto the points at the Estadio do Dragao, making the November 3 re-match at San Siro in two weeks' time particularly huge for Milan.
This is the Rossoneri's first season back in the elite UEFA club competition since the 2013-14 campaign, and it has been a tough examination to date.
Ibrahimovic promised Milan would fight for as long as they remain mathematically capable of making progress, claiming he remained "confident" of a turnaround, albeit realistic about their shortcomings on Tuesday.
"Today was our worst match in the Champions League," said Ibrahimovic. "Unfortunately we are still on zero points after three games.
"We must remember that this team is back in the Champions League after so many years and games like this will give experience to the whole group."
Speaking to Mediaset Canale 5, he added: "Players were missing, but this is part of football. I just got back and I'm happy to play."
Injuries and COVID-19 cases have left Milan depleted, but Ibrahimovic said: "I don't want to find excuses. In the next match we have to do better. There are three games left and, as long as there is a chance, we will try. I am confident."
The Swedish striker was used as a substitute, coming on just before the hour mark to become the first outfield player aged 40 or over to appear in the Champions League since Ryan Giggs for Manchester United in April 2014.
Ibrahimovic also became just the second 40-year-old to appear for Milan in the competition, after former defensive stalwart Alessandro Costacurta.
Milan head coach Stefano Pioli accepted his side were second best on the night in Portugal, praising Porto's efforts.
The Italian outfit have lost their last five games in the Champions League, but two of those came against Atletico Madrid at the last-16 stage eight seasons ago.
Pioli said of Porto: "They did better in terms of clarity, technique and how they were on the pitch. We didn't start well, we had to manage the ball better.
"There were situations where we could have created some more danger, but we didn't succeed in doing so. The performance wasn't the best. At this level, if you don't play with clarity it becomes difficult. They did better than us."