Kadri at least has it in him to be good defensively, he's less engaged this year but last year I thought he was solid on both sides of the ice. Kessel is what he is, he will never be more than a mediocre defensive player, and Raymond is okay on the PK but has way too many turnovers.
Kadri needs to watch Bolland and Bozak more, you do not have to be big to understand how to gain position on a player to win the puck battles, you do not have to hold the puck too long to make a play. He is very much a work in progress, he is still too weak down low, and he does not support his defence like Bolland, Mcclement, or Bozak do. Infact I see more defensive ability in Holland.
Kessel will likely never be a great defensive player, this is why he misses Bozak when he is not there.
He needs someone to get him the puck in the neutral zone and someone to cover up his defensive weaknesses.