Manchester united will play against big teams like Manchester City, arsenal and Barcelona. Manchester United performance is not poor, I think they are ready to play against any tesm with their form of performance.
Manchester City is not in the Europa League so Manchester United will only meet Manchester City in the Premier League or the FA Cup this season. And for Barcelona it is very clear that they will face Manchester United because the play-off match in mid-February will take place at Camp Nou where Manchester United will play away there against Barcelona. So Arsenal doesn't need to be discussed at this point because Manchester United and Barcelona still have to pay attention and need to fight better in the play-off round.