Milan head coach Stefano Pioli said his team "must not repeat" their defensive errors in Saturday's opening win against Udinese when they face Atalanta next week.
The Rossoneri got their Serie A title defence off to a victorious start at San Siro as they beat Udinese 4-2, but had to come from behind after conceding just 90 seconds into the new season when Rodrigo Becao headed in a corner.
A penalty from Theo Hernandez and an Ante Rebic strike quickly turned things around, only for Adam Masina to head in an equaliser just before half-time.
Brahim Diaz restored Milan's lead less than a minute into the second half, before Rebic wrapped things up with just over 20 minutes to go.
While Pioli was clearly unhappy with conceding twice, he did acknowledge that his side recovered and played some of the football that saw them win their first Scudetto in 11 years last season.
"I didn't like conceding so early; we have to start better," he told DAZN. "Then we played half an hour of great football, against a very dangerous and physical opponent.
"We conceded two goals from crosses, we had to work much better. We will review everything and deal with these situations better.
"Sometimes we are more determined when we defend high instead of in front of our penalty area. This is a serious mistake, and we must not repeat it next Sunday against an opponent like Atalanta."
Rebic scored twice, his first Serie A goals at San Siro in 482 days, having not registered a league goal on home soil since playing Genoa in April 2021.
"Ante is a very strong player," Pioli added. "He knows how to move, he knows how to tie the game, he has all the characteristics to be important. He worked for the team, he really gave us a lot, so congratulations [to him]."
New signings Charles de Ketelaere and Divock Origi made their debuts from the bench, with the former putting the ball in the back of the net only to see it disallowed for a foul by the latter.
Pioli was asked about De Ketelaere, a big-money arrival from Club Brugge, and suggested the Belgium international will need more time before starting.
"The [first] impressions are very positive, he is an intelligent guy," he said.
"He can read situations in advance... He still doesn't have the best possible condition [fitness-wise]. He was probably used to working between the lines more, we aim to fill those spaces with more players.
"There is no doubt about his talent, but he comes from a different league and will need the time to understand this football. "