Tbf I don't think Maguire's marking Wan-Bissaka - when you watch the replay it's actually Wan-Bissaka mistaking Maguire for a Southampton player and blocking him off. Maguire's still to blame though. Utd zonally mark and he was too keen to attack the ball that wasn't in his zone - even if Maguire doesn't get blocked, he's not getting to the ball because Matic's flick takes him out of the game. Lindelof probably should have done better filling in behind but the Southampton player was very clever putting himself between Lindelof and the ball.