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.