Let's look at the disallowed goal.
Zaha gets the ball and is covered by AWB. Lindelof is also close, Maguire is in the center tight to Ayew
Second frame - Zaha has carried the ball inside and is covered by AWB. Lindelof is also in close proximity and McTom is busting a lung to get close to the danger and can cover a pass to the runner (Milivojecic??).
Pogba is tracking the other runner. At this frame, and with TFM badly out of position, Maguire HAS TO COVER AYEW. He can't let him ghost into space, he's got to cover him.
Maguire is marking none. He has to cover the red circle and prevent the ball from getting there. He doesn't.
@UNITED ACADEMY Harry turns his back on Ayew so he has no Scooby where he is, whether he's strayed offside or not. He's no man's land and he's at fault for losing Ayew. Thankfully the goal was offside by a hair.