Of course, it is potentially, because he didn't get to the ball. Maguire was in the way of him so prevented his chance.
Why was Maguire in the way? Because he too was going for the ball, thus involved in the play.
You are right, the ref came to the conclusion based upon the rules. As they should be doing.
Read the rules....
2. Offside offence
A player in an offside position at the moment the ball is played or touched* by a team-mate is only penalised on becoming involved in active play by:
- interfering with play by playing or touching a ball passed or touched by a team-mate or
- interfering with an opponent by:
- preventing an opponent from playing or being able to play the ball by clearly obstructing the opponent’s line of vision or
- challenging an opponent for the ball or
- clearly attempting to play a ball which is close when this action impacts on an opponent or
- making an obvious action which clearly impacts on the ability of an opponent to play the ball
That didn't happen, so kind of irrelevant.
But if somehow Maguire miraculously realizes he was an inch offside and stops before getting in the player way, or getting near the ball, then he wouldn't be involved in play, so goal would have stood.
We see it with strikers. They will deliberately stop, sometimes put their hands on their heads to make it clear they are not trying to get involved in player.
Ask yourself honestly, if Fulham had scored this very same goal, what would your thoughts be on it?
Im guessing you would have been pissed that an offside Fulham player was blocking one of our defenders path to the ball.