This could perhaps be used as a reason and motivation for the Manchester United team in the FA Cup because it is true as you said that only the FA Cup is possible for Manchester United to win this season.
If you look at the strength of the Nottingham Forest team, it is not that strong and it should not be difficult for Manchester United to win this match.
I doubt if Manchester United will be able to lift the FA Cup this season, I count them out, looking at other teams that are in the competition Liverpool, Manchester city, and Chelsea and the rest of other teams that are way better in performance this season than the United team, likely for them they may end up being able to reach the semi-final stage of the competition.