Considering that Napoli are the most consistent Serie A team, I think Milan will really face big problems when they face them in the Champions League. I think it's true that the quarter-finals are Milan's best achievement this season, but for something higher I think it's something that would be very surprising. Since Milan were draw to get Napoli as their quarter-final opponent, that's when I thought that failure would be easy for them.
We might be able to make the results in the upcoming Serie A as a prediction for the UCL result. Before competing in the UCL, Napoli will face Milan in Serie-A. If Napoli can win this match, chances are that in the UCL it will too. Unless, Pioli implemented a different strategy that could give Milan win this matches. However, if AC Milan wins, I don't think that will continue in the UCL. I remember, Milan fans in Indonesia say, Milan has UCL DNA, but I'm not sure that the current Milan squad can be considered as still having UCL DNA.

the AC Milan vs Napoli match in Serie A might indeed be a benchmark for analyzing how these two teams will perform in the quarter-finals of the Champions League. But it seems that the majority of us have already seen that there is an opportunity for victory for Napoli. because as we saw in the Milan vs Udinese match. in that match Milan was really bad at making chances. even though Milan controlled the ball more but they were weak in making chances. even their defense is easier to penetrate by the Udinese forwards. so in the match against Napoli maybe we wouldn't be surprised if Milan conceded more balls.