Stefano Pioli hailed Milan's "mature mentality" as he acknowledged his team are going through a rough patch of form.

Milan scraped into the last 16 of the Europa League on Thursday, as a 1-1 draw with Red Star Belgrade was enough to seal an away goals victory from a tie which finished 3-3 on aggregate.

Franck Kessie's early spot-kick – his seventh penalty goal of the season in all competitions – saw Milan through, with El Fardou Ben's equaliser proving to be in vain.

Gianluigi Donnarumma marked his 22nd birthday with a sensational save to deny Red Star a second, with Marko Gobeljic then seeing red as the visitors' comeback efforts were fruitless.

It was the first time Milan have progressed from a two-legged European tie having not won the first game since 2006-07, when Carlo Ancelotti went on to guide the Rossoneri to their seventh European Cup.

However, it is now four games without a win for Milan, who have lost their last two Serie A fixtures, including Sunday's damaging 3-0 derby defeat to title rivals Inter.

Pioli, though, insists he has faith in his squad to overcome their loss of form and learn from the mistakes they are making.

"The team might be young, but it has a mature mentality," he told Sky Italia. 

"Clearly, we are not in our best period of form. The many victories gave us energy and confidence, so now those elements are lacking a little bit.

"It's a pity we didn't control the situation better, with a little more precision in our passing and our choices, but the important thing was to qualify.

"At this level, it’s about details and the tiniest margins can make a big difference.

"If we think about our season, it's still full of positivity. The season doesn't end here, winning the Europa League does give you a place in the Champions League.

"We are making more mistakes and aren't as solid as we were in the past, but it's a very important stepping stone and it's crucial that we managed to get through."

Asked to pinpoint why his team – who went unbeaten in Serie A this season until January 6 – have dropped off, Pioli said: "I do think perhaps there's some psychological fatigue more than physical.

"I try to stay with my players throughout the game, give them the right boost, but perhaps they are starting to feel the pressure.

"They shouldn't, because we are having a remarkable campaign and going through difficult moments will help us become stronger in future."