If we look at the Atletico Madrid - Manchester United matchup, we can say that it is going to be a very competitive matchup for sure. Because I don't see a big difference between these teams now. Maybe Atletico Madrid's form lately hasn't been better than Manchester United. But if we take their general performances into account, it is more visible that they are close to each other. If I'm to favour one of them, my choice would be Manchester United.
If there's one thing I've come to know, it's the fact that Athletico performs well when they're set in a position like this to play against a team around the same level as they are. It has happened that way when they were pitted against Liverpool in the past so we expect the same levrl of competitiveness when they take on Man Utd. Also, Man Utf needs to bring in their A-game because Atlético mafis no Atalanta or Villarreal.