Nottingham Forest has the third best defense in the Premier League and Manchester United has one of the worst attacking lines. Yes a draw is of course possible, but Nottingham is the one team that was able to beat Liverpool, they beat Man City and they tied the game against Arsenal. Times are over that clubs like Manchester United take them for lunch. Remember 1st of February when they taught Brighton a 7-0 lesson?
Though this is football and anything can happen, but due to their head to head record, am very convince that Nottingham Forest have what it takes to beat Manchester united in this game, because in their last five meetings, Manchester united have won three while Nottingham Forest have won two, and in their immediate past meeting, Nottingham Forest won by 2-3 at away, so their is a higher probability of Nottingham Forest winning this one here again unless something unexpected happened that changes the cause of the game to the favour of Manchester united, so my prediction of this game would be a 2-1 winning in the favour of Nottingham forest since they are playing at home.
Some will call Nottingham the surprise team of the season and maybe some saw it coming. I agree with those saying they are a surprise because their weak point last season was the defense. Now they have got the third best defense in the Premier League and really, this is not negligible. The Premier League is strong and keeping the goal count against yourself low means a lot. Nottingham Forest has demonstrated this several times now that they can be on par with the best teams in the league. Therefore I think you are correct here that Manchester United is the underdog in that game.