Slot continued his 100% record this weekend as Liverpool triumphed over their biggest rivals and kept their third consecutive clean sheet of the season.
United were stunned as Luiz Diaz scored twice before Mohamed Salah finished the game off with a sumptuous finish which emptied Old Trafford before full time.
The Dutch manager spoke about his tactics and how he adapted to United’s style of play as the game went on.
"Today we showed a lot of moments where the forwards tracked back so aggressively and that is what we need if you want to win these games.
"We said (at halt-time) that United were going to press differently, so we tried to prepare for that. But we also showed them (the players) three moments where in one situation that led to a chance for Mazraoui, Luis was not tracking back as he should, and a moment when Trent played a sloppy pass back to Alisson. Those moments can change the game at Old Trafford.
"You need to be focused and concentrated in every second if you want to win here."
He spoke about how he wanted to press United and put pressure on certain players to control the game.
"There is not a specific gameplan from the back, we always want to press the opponent high," the Reds boss added to Sky Sports. "The gameplan was when we had the ball, normally Dominik (Szoboszlai) played from the right, but today we played him from the left.
"Last season they (United) were man-marking on the midfield, this season they are on a nine and 10 press with a 4-4-2. That's different. In my opinion they are working harder when the ball is played through them so they run more - that is at least what I saw in the first games from them.
"United often play with their full-back really high and Casemiro drops between them, so if you can keep the ball and keep Mo Salah and Lucho high then you are constantly in a one-v-one situation. Then you need midfielders who can run, and we had three of them who kept on running and were aggressive enough to win in their duels.
"Today we showed a lot of moments where the forwards tracked back so aggressively and that is what we need if you want to win these games."