I didn't see anything special about Manchester United's game, they were just luckier because the Liverpool coach was too hasty in making substitutions. Basically, Liverpool had enough control of the match and they were even able to maintain their lead until the end of the second half before the equalizing goal occurred. So Klopp's mistake in rotating players gave United players more freedom and more confidence because they were able to force their opponents into extra time. So in my opinion it would be an exaggeration if that was the best game from the Red Devils.
In the world of football there is no such thing as luck, in my opinion this is the result of the Manchester United team's hard work with excellent passes which were then put to good use by the Manchester United players resulting in the winning goal for Manchester United. It's true that Liverpool played very well, but they played too hastily so they couldn't take advantage of all the opportunities they got when they were in the Manchester United goal. In my opinion, this is not the Liverpool coach's fault, in fact Kloop gave his squad a strategy that looks very good. It's just that Liverpool's defense is too focused on the front so they don't pay enough attention to their players from Manchester United.
Yes, it was Man Utd's hard work when facing a strong team at the moment, but I think Liverpool didn't put all their energy into this match because Liverpool was more focused on the EPL and UEL competitions, so even though they lost in the quarter-finals of the FA Cup, it doesn't change the possibility that Liverpool still strong and this defeat against Man Utd can be a lesson for Liverpool not to underestimate teams that are not very consistent because sometimes teams like that can give surprises, Man Utd finally qualified for the semifinals and I think Man Utd can reach the final and fight Man City to win the trophy.
Indeed Manchester United game against Liverpool in the yesterday was a big win for man United, but then the bigger question is that will just a match against the top club make the bug difference, if you have being's following, both in the premenre leagues and FA and other leagues, Liverpool have being doing well in this this and just losing a match is not a big deal that couldn't be taken as means to get to something better outcome since the league is almost close to it end.
Anyways we have few games to go and at that Manchester United and Liverpool still have more games to prove themselves, I guess Liverpool in under high pressure right now, because of their position in the Premier leagues which their Don well from the beginning but at the moment their have lost a few point which drip Liverpool back to second on the leagues table right now.