The midfielder was superb in Sunday night's 6-3 win in London.
Slot said: "I haven't seen all of his performances because this is my first year and his second. I just referred to the Man United game and I think he was outstanding over there as well.
"If you score so many goals, that's always what stands out, but last week against Fulham, for example, if you look at the work-rate he did over there. It was a very good performance from him.
"I think it wasn't his first great performance, but it was definitely one of his best, yes."
On working with two-goal Mohamed Salah, Slot also stated: "You have an idea before you start working with players, but the best way to judge them – or to experience them – is when you start to work with them. I think Mo shows what he has shown for so many years at Liverpool, that he can score a goal and the team just has to provide him to come into these positions where he came today many times.
"Then he is able to score goals or provide assists. Apart from him being a very good football player, I think he is a very likeable person to work with as well. He is always there to help his teammates. He is always down to earth, so tomorrow – or even now, I think – he will work on his recovery.
"He understands what it takes to be at this level every three days and he is working really hard for that. So, that maybe impressed me most – that a top player like him understands how hard he has to work for the quality he shows."