To be fair, without a wider view, it is actually difficult to tell if that does show him to be clearly onside given the Coventry attacker was actually stepping away from the United goal and the United player was stepping toward it when the ball was played. That means the defender could be much closer to the United goal by the time the attacker gets on the passed ball.
Still, even so, I do think the video highlights the issues with the current offside assessment system (namely, how it can be manipulated in instances of very tight decisions).
try and pause it on about 5 seconds the United is so far in front of the Coventry player