The hosts went into halftime 1-0 down to Saints through a Will Smallbone goal, but fought back after the break through Darwin Nunez's equaliser ahead of two successful penalties from Mohamed Salah.
Slot was again in the stands for the game as he continued to serve his suspension.
He remarked afterwards: "I didn't give them compliments at half-time, I can tell you. Maybe it was because I was sitting up there instead of being at the line because I know from experience, not that I've been suspended before, that when you watch a game over there, you always feel like, 'Maybe I can even play in this game.'
"But if you are then at the line it's always more tempo. But I don't think I was wrong this time if I said at half-time that energy levels were far, far, far too low. That is what had to change and that's why we made three substitutions just to, apart from bringing in quality, also create something.
"Because nine out of 10 times when you take three out, the other eight are like, 'Ooh, something else should happen.' That's the only thing I could come up with at half-time to create something different for the second half."
On Nunez, who's goal shifted the momentum in Liverpool's favour, Slot continued: "That's also the life of a No.9 – you go from missing a chance to scoring an important goal to then, all of a sudden, missing a chance again.
"But I think today, especially in the second half, you also saw the work-rate he can bring to the team. But the main thing that happened was in the second half we started to play in a different tempo – not only without the ball but also with the ball.
"The first goal, if you look at Lucho (Luis Diaz), how he takes that one-v-one on, I don't think I can select a clip where that happened in the first half. In the second goal, Ryan Gravenberch pushed up all the way at the 18-yard line to win that ball which led to the foul on Darwin, whereas in the first half every ball that fell out they could just pick up the ball and start to play.
"The energy in the stadium for the players and the fans in the second half was completely different than the first half."