I don't think Kadri has ever been as good as some people think.
When he was centering the shutdown line, he could shadow one player fairly well, and be aggressive physically, because Komarov was the defensive mainstay. As soon as they were separated, and even more with Komarov gone, his defensive and physical game has disappeared, even with easier matchups.
His offence is primarily scoring, and most of that comes from the PP, or, for a while last year, from Marner's amazing ability to set him up. His PP work reminds me of Kessel's with Pittsburgh - the fifth best player is the one left open most often.
He has improved, but is still not a very good passer, and is prone to making low percentage ones. Fortunately, most of those occur in the offensive zone.
Short of putting him back with Marner and hoping they can outscore their defensive problems, or trading him for a 3-4 defenceman (or in a package for better), I'm not sure what to do with him.