I am sure Nottingham let MU win the match. Nottingham want secure their position on champions league zone because it is still vulnerable to shift Chelsea, The City, and Newcastle. Their performance is good to hold that position, but, in the remaining 9 matches something could be happen to shift them on top premier league. We don't know which players are likely to get injured so that affect their performance. If the team are still complete, and Nuno Santos keep running his strategy like he did before, I am sure Nottingham position are still safe until finish. MU is not in good condition, They can take advantage of this opportunity to increase points, and secure on 3rd position.
I also think Nottingham let Manchester United win on purpose. Nottingham wants to keep their spot in Champions League zone but they know Chelsea and Manchester City and Newcastle can still catch up. Nottingham has played well so far but anything can happen in last 9 games. If their players do not get hurt and their coach Nuno Santos keeps doing what he is doing I think Nottingham will stay safe. Manchester United is not playing their best right now so Nottingham can try to get more points and keep their third place spot.