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.
It can be said that this match was unlucky for AC Milan, because AC Milan played quite well, it's just that Feyenoord's defense was very disciplined in this match, making it difficult for AC Milan to penetrate the opponent's defense area. In addition, for some reason I saw Santiago Gimenez still looked awkward when playing against his former team, is this just my feeling but I hope and must change in the second leg later at the San Siro. In the first leg AC Milan lost, I can still understand because their defense was very solid, besides that even a team like Bayern Munich was broken into 3 goals by Feyenoord.
AC Milan in the 2nd leg must be better and must be able to break through the opponent's defense by scoring a quick goal like Feyenoord did in the first leg in order to qualify for the last 16. Moreover, it is likely that Feyenoord at the San Siro will play defensively and rely on counterattacks because they are already 1 goal ahead.
I see AC Milan as still a big team and has a strong mentality, especially since they have the most Champions League trophies after Real Madrid. So I think AC Milan can definitely bounce back in the second leg.