That is indeed a crucial match for both teams but more for Nottingham Forest. If Nottingham Forest succeeds in qualifying for champions league next season, that will be a big achievement for them, that is why they need to win as many matches as possible. The champions league spot is still a very big fight between Manchester City, Nottingham Forest and Newcastle but just like you said, Chelsea have a very high chance of beating Leicester to enter the third position depending on what the outcome of Nottingham Forest against Manchester City is as either of them that wins the match will be third.
In that match at Nottingham Forest against Arsenal, it seems to me that the Nottingham Forest may also get points from this match. But this match at Manchester City and Nottingham Forest can be much more important. It should be noted that Manchester City has won very well in the FA Cup, in fact they will try to win the trophy there by scoring three goals in that match. So my idea is that if the Nottingham Forest can strengthen their defense then the most difficult thing for the city will be. Yet I am convinced that the Nottingham Forest will not play bad in the next match.