He's too weak and inexperienced to play D at the NHL level. If he gets untimely delay of game penalties as a winger shooting the puck over the glass how much will that increase playing D? Plus his slap shot is too weak and he will give the puck away more. Keefe should just have said "no....you just don't have the right stuff for that, Mitch, so get back to your wing!" I give Marner credit for trying though. If he likes trying something different, maybe try him at C for a few practices and games with Tavares as one of his wingers.