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.
Both teams are currently experiencing the same problem in the league, which is both have not found their best performance, but even so I think the match will be very exciting, because both teams who are currently slumped in their league will certainly show their best performances to secure the chance to qualify for the the next round, I will not bet or choose one team in the match because it is very risky in my opinion.