There was bad luck that they had to feel in this match so that they could not get a satisfactory result. If you look at the way they played, they should have gotten a draw. However, that is the reality, we cannot predict what will happen to a team even though they performed better than the opponent they faced. There is still a chance for them in the second leg later, being 1 goal behind is not a far distance, they can reply when playing in front of their own supporters. With the note that they must be able to play well.
I know that Milan still has the chance of qualifying with the return leg match in San Siro, because they will make proper use of playing at home to oppress their opponent and win them hands down. One goal is not a big deal to get, Milan has sharp strikers that can pump in goals into their visitor's post. I hope everything goes as expected. If Milan finds difficult to score just like in the first leg, it means Milan will be out of the competition.
To make everything go according to what is expected, they must look for an early goal after the match starts. They must be able to take advantage of the opponent's situation which will certainly be a little disturbed by the atmosphere created by their supporters in the stadium. At least that is one thing that can be done, because when playing away, usually the opponent will start with a little nervousness. We also know that AC Milan has a militant fan base too, so they can take advantage of that moment. Preparation must also be mature, they must also have several strategies that can be used in difficult situations.