Maybe you mean the Premier League is not the Champions League because both teams are not in the Champions League competition this season. And what we need to believe in this case is that each team will always try to secure three points after the match is over because there is no story of intentionally or letting the opponent win as long as the team has the ability to prevent it. The Nottingham Forest vs Manchester United match could be quite exciting because Manchester United is usually more enthusiastic when facing a team that is already at the top.
I think he's right by his statement because Nothingham Forest is currently the third team on the premier league table which means if they finished in that position they would play in the champions league next season. If I understand correctly, he meant to say Nothingham Forest can not deliberately throw away their chances of playing in the champions league by giving Manchester United a soft landing. Nothingham Forest performance this season has been very impressive against the big teams and as such I don't think United really poses any threat at the moment to defeat Nothingham Forest but if they manage to get the victory, it's fine then but I really doubt the possibility.