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.
My favourite team for this matchup will be Manchester United. But it is really early to talk about which team can advance to the next round. Because there is still a long time for the first leg to start. Until that time, everything can change about these teams. Atletico Madrid are in a bad situation for example. But they are still a strong team that they can fix their problems in this time period. And Manchester United have been newly recovering. We will see if they will be able to get better until this matchup. If they develop their game more and more, they can fight really hard against Atletico Madrid.