He’s the center, McDavid is not his responsibility. Hockey doesnt use man to man. His job is to support the defenceman in the corner who got beat so Marner doesn’t walk out and score. In this case it was probably the wrong decision since he was late to Marner too. But in his defence it’s a quick play and there’s no way for him to know that McDavid had popped out into the slot behind him. Was just a great play by Canada.