Overall it is true that Nottingham Forest are much more superior, especially the match will take place at home so their motivation to win the match is much greater, especially since there is something they want to achieve in the Champions League. Nottingham Forest is one of the clubs that is quite surprising this season because they are able to play well to reach the top three positions in the standings.
That is different from Man United who this season has experienced a decline and they have lost their best form which is much worse than the previous season. But even so in this match it will not be so easy for Nottingham Forest to achieve victory because Man United themselves will also try to get points.
One of the motivational factors that has helped Nottingham Forest this season has been their intention of playing in the champions League come next season and this is the reason why the team has been very formidable and defiant whenever they are playing any match, unlike Manchester United that has always has to struggle in the domestic league but seem to be doing well in the Europa League, besides that the club has refused to bring in new players to replace some of the old players in the team, which is one of the reasons why the team is where they are on the table and may remain so if things do not change in the dressing room before the end of the season.
I won't totally agree to your statement that "Manchester United has refused to bring in new players to replace some old players" is the reason why they are were they are in the league table. That's not totally true, Manchester United has been one of the most recruiting teams in the premier league so far as I know. There is no window that passes that Manchester United team don't bring in players since Alex Ferguson retired. Even Amorim that came in this season bought a player during the January transfer window. I don't think it's lack of recruitment that's the problem of Manchester United. Maybe they are always scouting the wrong players for their team I don't know. But the funny thing is that the moment these players leave Manchester United and go elsewhere they do better there than they did in Manchester United. I don't think anyone can directly point out what is Manchester United problem. But in all they need improvement.