If in the match later Osimhen will still be absent due to injury, then Milan's chances will be slightly superior in this match. It was clear from the meeting between the two of them in the domestic league, how difficult it was for Napoli to score a goal in that match and they had to be willing without being able to score a single goal. What is clear is that Napoli's defense also needs to be repaired ahead of the first leg match against Milan in the QF round later, it could be the same destruction as before if Spalletti fails to fix it.
Osimhen injury isn't that bad and I think he will be ready to be back on the pitch when he takes on AC Milan in the UCL. but there is no definite certainty because anything can happen meaning that if the recovery goes well then he will return quickly to the team but if there are problems during the recovery period then I am sure he will be absent when facing AC Milan in the UCL.
for ac milan maybe the absence of osimhen will be used as best as possible for them to play aggressively like they did in the serie A when they beat napoli with a score of 0-4. the first leg will take place at the sansiro which is an advantage for ac milan to win if they are able to play well.
To be honest, I also hope that Osimhen can appear to strengthen Napoli against Milan later. We know that Osimhen has been absent twice in domestic league matches and that is quite doubtful for Napoli. But if indeed Napoli gives long enough time to recover from injury, I think it's a very good decision. I already miss the Napoli duo Osimhen and kvaratskhelia, I hope to see them both contributing goals and assists as usual.