I watched a lot of Lille last season. In a nutshell, I would say he is a decent footballer but a poor defender. I couldn't tell you how many times I saw him roasted. On the basis of his lack of pace and agility alone, I think he would be a terrible fit for us/PL football.I didn't know that. Bad news.
I heard a lot about Çelik during the 2018-19 season (his best so far). He did well playing behind Pepe. He seems to be good on the ball and he is known for solid defensive numbers. I found this grafic very interesting considering what we need from our RB - passes into the final third, progressive distance, and again, defensive qualities. I also believe that he is good at crossing, which is not measured there.
If you could speak more about his qualities and problems, I'd like to hear.