Ngumoha started and scored in the first-half of their 2-0 win at Anfield, with Mohamed Salah also scoring before halftime.
Asked about the teen, Slot said: "I think I’ve said a few weeks, maybe months ago, one or two months ago, that his playing time would increase because he got stronger and stronger, showed more and more when he came in, but also in training sessions, that it wasn’t only a nice trick, but there was more and more end product to what he did.
"There was more and more power to what he did, which is needed if you play against the best defenders in the world."
On starting against PSG in their Champions League quarterfinal second-leg this week, Slot was adamant.
He also said of Ngumoha: "Yeah, I think he’s ready. Now the question, of course, is: can he do this two days later again? That’s another question. But would he be able to play and perform at that level? Yeah. I play him because I see him as one of the players we are having.
"It’s not like he’s less or in the beginning of the season he was a young player getting some experience with the first team. But now he’s just someone I can pick for any game. So, also the one on Tuesday.
"But I think in general, that’s not to say about Tuesday, but in general expecting from a 17-year-old that he can play three games in six days, you could question if that’s realistic, especially for the ones that have been there in Paris and saw that intensity. That’s almost for every player too much, as you noticed today in my line-up, that I was aware of how intense two days ago was and how intense two days from today will be again."