The Reds were beaten 3-2 on the night, seeing them drop into sixth place on the Premier League table.
Slot said afterwards: "You are always hoping if you win a game of football during the week, although it is only two days' rest when you come over here, of course you are hoping for a better result, but I was also hoping and expecting a better performance, because the performance was far from what we are used to. Even if we are losing, our performances have been better than tonight."
On what he was most upset about, Slot continued: "Doing the basics right. Although if you are 1-0 down, 2-0 down, 3-1 down with the energy in the stadium, that is definitely helpful for them, and they are a very good team in winning duels and winning second balls, so you have to give them credit for that as well, but of course it is hard to win a game of football if the other team wins more duels, more second balls than you - especially if that is also something they are playing off. Again, like I've said many times, it's also difficult to win a game of football if the set-piece balance is in their favour."
Slot conceded it was the worst performance he's seen from his players since his arrival.
He said, "Yes, I think it's up there in my time here in terms of losing a game of football because normally when we are 2-1, 3-1 or 1-0 down... by the way, after we went 1-0 down, that was the only 20 or 25 minutes I really liked from our game today.
"Unfortunately, we had a great moment with Florian (Wirtz) that didn't lead to a goal because you need to then have a little bit of luck and those moments can bring you back into the game and can give us the energy.
"But, after 30 minutes, I think they took the game again with long throw-ins, set-pieces and other moments where they were just really good. Again, please don't focus on what I've just said because they did many things well."