The Merseyside giants have opened up a 2-0 advantage over their fierce rivals after a dominant performance at Anfield.
However, while McManaman has praised Liverpool's display, he feels that they should be taking a bigger lead to Old Trafford next week, although United stopper David de Gea did pull off several tremendous saves to keep his team in the contest.
McManaman said: "They were excellent tonight. The only thing that they didn't do is that they didn't score enough to actually wrap up the tie.
"If United put this type of team out, Liverpool will go out and score goals there next week. This is the most disjointed Manchester United team I have ever seen. I didn't know where anyone was playing tonight.
"Poor Marcus Rashford played at right-back and got dragged off. They had players all over the place playing in the wrong positions."
Victory for Liverpool ends a four-match losing sequence against United, a run stretching back to March 2014.
On that occasion, Liverpool emerged victorious by a 3-0 scoreline but they will not have to pull off that kind of result in the reverse fixture next week after leaving United will it all to do to remain in the competition.
Both clubs could need to win the tournament in order to qualify for next season's Champions League.