It takes time for all of that to form, it is not easy for a coach to form a solid team, the players in the AC Milan squad are good enough, it's just a matter of time, a 1-0 defeat is not a problem in my opinion if in the second leg AC Milan is able to turn the advantage of the match around, because this is their moment to survive in the next round, even though Jaoa Felix has performed well but not optimally, that's why it takes adaptation from all players who need him to play more to find out the game that Sérgio Conceição wants.
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.