City won 4-0 at the Etihad, with Erling Haaland hitting a superb hat-trick.
Slot remarked afterwards: "It's very disappointing to be out, not only in the manner but also the result and the score. Another big disappointment for us."
On what was missing from Liverpool today, Slot continued: "What has been missing for large parts of the season, converting our chances into goals. I think it was an even game up until they scored the penalty and the last time we played here we conceded just before half-time and again today.
"We get a penalty and we get a chance but that doesn't tell me anything. The first 35 minutes was the sort of team I would like to see but the 20 minutes after that, we have to defend so, so much better than we were doing today.
"Not nice to go in at 2-0 just before half-time, not helpful for your mood especially after the season we have had. That was really hard to take. The only good thing was that we didn't concede more."
On Mohamed Salah's penalty miss, the Dutchman insisted: "I wouldn't focus on one player, I would focus on the team and that probably sums up us today but also maybe large parts of the season.
"I think it is the second one he missed and Dominik Szoboszlai also already missed a penalty so it's not only that we miss big chances, this season we have missed a few penalties. In the first half there was a chance for Mo when we played the fast counter attack.
"Unfortunately that didn't go in and you have to score goals if you want to have a chance to win the game."