If Manchester United doesn't take advantage of this opportunity and can't win the match against Copenhagen, of course it will be difficult to get through the group, but if they get a win it might give them a small chance to get through the group, even though it will have to be a very hard fight because they have to beat Munich and Galatasaray in the next match with a possible aggregate provides a gap and opportunity to qualify.
I don't think that Copenhagen is a club that can be underestimated because they can definitely make it difficult for Manchester United to win, but if they really get up and play consistently they can definitely beat Copenhagen easily, you don't need to win big, just win narrowly 1 - 0 is more than enough, especially when playing at home as hosts, Manchester United must play as hard as possible to get the win if you really have a strong mentality.

Manchester united would revive their hope of qualifying for the next round if they best Copenhagen home/away, however Copenhagen is not an easy club to play against with the style of football Manchester United is playing i doubt if United would be able to win them in both games maybe they could win one and draw the other cause they don't even have a good defence line that would play week in an out due to numerous injuries from that position this season, and I feel that game could cause more injuries for Manchester United.
It's too bad Manchester united has reduced to this level where they'll be hoping on a 1-0 win against Copenhagen, if it were in Sir Ferguson's era they would comfortably beat Copenhagen home/away and could probably be leading their group table, Erik Ten needs to change his strategy and trust in players like Gernacho and Erikson.