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.
For sure I don't think Manchester United have a chance to win the FA cup. The way things are going they are likely to end the season without any trophy. They don't have what it takes to win a trophy this season. Manchester City are gearing up to repeat their last season feat by winning the tribble and Liverpool and on course to win the quadruple. These teams are more hungry for success compared to Manchester United who are yet to identify there pattern of play.