Milan drew 1-1 at home to Red Star to progress 3-3 on aggregate.
Pioli said, “The team might be young, but it has a mature mentality. 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," Pioli told Sky Sport Italia.
“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 against a team that caused us problems over both legs.
“In the first leg, we stopped looking for the third goal and rested on our laurels. We didn't do that this time, we had the chances with Rebic and Ibrahimovic. At this level, it's about details and the tiniest margins can make a big difference."
He continued: “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.
“I do think perhaps there's some psychological fatigue more than physical, as we started the campaign in the preliminary rounds. I try to stay with my lads throughout the game, give them the right boost, but they are a responsible side and 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.
“In the past, we were able to control the game more, pass it out from the back, create chances quickly in attack. Now the slow movement is seeing us get stretched out more. I think it's partly because opponents know us more now, partly we aren't moving as quickly. We need to dominate the match more, make more co-ordinated movements and more aggressively shaking off our markers."
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