A 4-0 result seem to me as an exaggeration for this match. 
Milan will feel especially Ibrahimovic's absence in this match but I don't think they will lose this match with too many goal difference. I feel confident about Milan will also score but Man Utd will defeat them 2-1 or 3-1, in my opinion.
Manchester United are now stronger than Milan, all the more so in a home match they have to defeat their rival. I think Milan will play in this match with the expectation of a draw and will try to score a goal on the opponent's field. I'm not sure if they can do it. Milan will feel especially Ibrahimovic's absence in this match but I don't think they will lose this match with too many goal difference. I feel confident about Milan will also score but Man Utd will defeat them 2-1 or 3-1, in my opinion.The hardness of the match is very pronounced, even the 'shot on goal' done by Milan is far more than Man U. before the 90+ minutes are finished, Milan's chance to make a goal is still big.
First half both teams played equally although United had more ball possession than Milan.
The match turned 180° in the 2nd half, Milan did their best in the 2nd half with more ball possession and more goal attempts.
I'm not really sure what is happening with United in the 2nd half but I have to say the last 3 substitutions by Ole made their game even worst.