Although I know Manchester United played a very good match against Liverpool in the FA Cup quarterfinal, which was one of their best performances this season, that shouldn't have given us hope that Manchester United can play that well in their next match. Although Manchester United has players who, when they are serious, can play very well and win a match, sometimes their performances look like an academy team or a small team that doesn't know how to play football. For that reason, I don't trust Manchester United as much.
Manchester United did perform very well when facing Liverpool in the FA Cup yesterday, because this match was played at Manchester United's home ground so the game looked very dominant in possession of the ball. I'm also not sure Manchester United can win against Liverpool in the second match, especially since this match will be played at Liverpool's home ground. Manchester United will definitely have great difficulty in attacking, because Manchester United's performance still doesn't look consistent in every match and they always make a lot of mistakes, especially in Manchester United's back line.
You are absolutely correct Liverpool played excellently well against Man Utd in their last FA cup match unfortunately the team was very unlucky after series of missed chances and dominated possessions, from my own point of view Man Utd hasn't play any spectacular game that day but they were just lucky and I doubt if they would make top 4 in the league due to their inconsistency in their performance, meanwhile their next fixtures are against Brentford away, away to Chelsea and home against Liverpool these are matches I can't vouch for Man Utd to win all those three matches unless they can spring a surprise of which I don't think it's possible going by their present form.