Mohamed Salah and Harvey Elliott struck for the Reds, with Aissa Mandi sent off for Lille before Jonathan David scored.
Slot said afterwards: "The privilege of managing Liverpool is you have more than 11 good players. You know how tough this competition is.
"It's up to the players to be ready if they get a chance. His (Harvey Elliott's) goal was a lucky goal, but his performance was good.
“We wanted to win this game, but we were also aware of the fact that we had to manage the load because we would have been really, really, really unlucky if we'd have lost twice and still not been in the top eight.
“As we know now, I think 18 points would have been enough to play top eight, but now with 21 there's no discussion anymore.”
Asked about Curtis Jones’ fitness, Slot added: “There was a moment in the game where he felt (something) and he said let's play for five more minutes and then see how it is, but he said he had to go off at half-time.”