Because of the way Nottm Forest has performed so far in their match this season, many people will be happy to see them if they finally make it as the top team that qualifies for Champions League next season. They look unstoppable in achieving their aim if you ask me because they have won and draw against strong teams better than Manchester United. Manchester United can stand in between if they face them in few days to come. At least they are the host team, they have the advantage to win the match against Manchester United.
With the high team spirit of the team I don't see Nottingham Forest leaving the top 4 any time soon and some of the bigger teams are definitely afraid of competing with them because they know that things have changed for this particular team that every other team do win them like is nothing, even Manchester City can't stand them or should I say Liverpool or Arsenal? They are unstoppable like you said and if they finish 3rd or 4th and book a spot in the champions league it will be their greatest achievement of this very campaign and the Manchester United you are talking about Forest are more organize than the Red Devils so I feel is going to be an easy 3pt when both team clash horns.
The last time I checked I think Nottingham is indeed a roll with their high team spirit and with their impressive performances this season, I think their are currently sitting on 3rd place in the premier league table standing with 54 points from 29 matches I think, their recent win against Manchester United with an score of 2-3, this chances will boost their confidence.
Why it's very hard to predict their upcoming matches, but from past analysis I think Nottingham forest has already beaten them one's this season, however Manchester United will likely be looking for revenge and I think the match will be an contest one between the both teams only if Nottingham forest can maintain their momentum they could potentially achieve their greatest chances of winning this season.