Stefano Pioli praised Milan's resilience as his side came from two goals down to beat Celtic 4-2 and qualify for the last 32 of the Europa League.
Tomas Rogic and Odsonne Edouard put the Scottish team 2-0 up inside 14 minutes, but Milan stormed back through goals from Hakan Calhanoglu, Samu Castillejo, Jens Petter Hauge and Brahim Diaz.
The result, combined with Lille's dramatic 2-1 win over Sparta Prague in Group H's other game, means Milan have sealed their spot in the knockout stages.
Pioli – back in the dugout after missing three games with COVID-19 – lost experienced defender Simon Kjaer to injury in between Celtic's goals, and the Milan boss was pleased with how his young side overcame those early setbacks.
"The lads wanted to put me to the test, to see if I'd fully recovered!" Pioli joked on Sky Sport Italia.
"We had started well, but those two distractions caused problems. It can be a teachable moment to help us grow. Even when 2-0 down, we have to keep playing as a team, which is what we did today.
"I am glad that even without Kjaer, and with such a young line-up, we didn't lose our heads at 2-0 down. Celtic proved they wanted to get an important result and make a good impression at San Siro."
Daniele Bonera was in charge of the first team while Pioli self-isolated after testing positive for COVID-19, with the former Fiorentina boss admitting his time from the club was difficult.
"Those two weeks felt like an eternity," he added. "Aside from the games, it is a genuine pleasure to spend time with these players and my staff. We want to continue like this and be ambitious."
Diaz took his tally in this season's competition to three goals, then admitted Pioli's return to the sidelines give Milan's players extra impetus for the game.
"Pioli's return was very positive," he said. "His staff allowed us to work well, too, but his presence gives us an extra boost. We are satisfied with the performance."
The on-loan Real Madrid forward has been limited to just six starts across all competitions this season, yet he insists he is happy at the Serie A leaders.
"I am very happy to be here, as I feel really good at such a big club as Milan," he said.
"I play wherever the coach wants me to. I know that I have to improve and I'm very happy to be doing it at this club."