Even if Chelsea continues to slip, and Lille improves its game, then 99.99% you can be sure that Chelsea will still be the winner of this pair. Lille is very bad this season - 11th in the weakest of the top leagues. Making it to the playoffs from the Champions League group stage is easy to explain - they had the weakest group (possibly one of the weakest in history) and scored only 11 points. I think Chelsea will never get nervous about this confrontation and will easily succeed here.
Yes, basically on paper Chelsea will qualify, no matter the bad momentum that may occur in the Chelsea team later if something happens, basically Chelsea have a great chance to qualify. At least that's what I also believed before, Chelsea have a great coach but still, they won't always get the results they hoped for. I only consider Chelsea's situation as it is now at least in the last 5 games yesterday, the results were not so good and it looks very difficult for Chelsea to get out of that pressure. For this reason, I made a thought and became worried if a situation like this happened again when Chelsea played against Lille, at least it would make me doubt it only if bad conditions like now happen again.
The underdog always has a chance, I agree with you. Even now, based on the odds that bookmakers give for Lille's success:
UEFA Champions League 2021/22 - Lille OSC reach the Quarter-Final?
No - 1,25
Yes - 3,60
it can be seen that, in their opinion, Lille has an approximately 25% chance of winning this confrontation. By the way, I am surprised because I thought that Lille's chances would be estimated as about twice as bad.