While this tie is still alive, with Manchester United restricting Barcelona to a single goal at Old Trafford, the ball is very much in Barca's court. The La Liga giants have not lost a home game in the Champions League since 2013 and, for all their improvement under Ole Gunnar Solskjaer, it is hard to see Man United being the team to end that unbeaten streak.
The main target for Valencia is Europa League. In La Liga they are not going to try to make the push for the CL spot. I am sure, that they will try to do it by winning EL.
At the same time Atletico is fighting with Real for the second place and losing points at home isn't the best idea. Valencia doesn't have lots of chances.