Luis Muriel's second-half penalty sealed a 1-0 win for the visitors at San Siro and stretched Milan's run to five games without a win in all competitions and a fourth loss in a row.
The 42-year-old admits it is tough to explain Milan's malaise but is adamant that they can still qualify for the Champions League or Europa League this season.
"I saw a great game from my team but we needed to speed things up in the second half," the head coach told Sky Sport Italia.
"We could have done better in the final third and from the point of view of our opportunities. But I can't blame the players - they put in a good performance.
"It's a difficult period to explain: we're sorry, but football is like that.
"We have to stay calm to continue on this path. We're on the right track. I think our growth is clear: we dominate matches even against the major teams.
"We need to get into Europe and we believe strongly that we can. I believe it."