Quite obviously ain't the match ball. As the ball goes in, ball boy behind the goal sheepishly knocks the doctored ball (with probably a thread attached) ready. The "ball" also deflates quite oddly on the close up which is understandable if you've just punctured it with a thread...
Lastly the ball is kicked into the stand to remove all evidence
Ingenious goal celebration but I've seen a number of Albion players literally disappear on the pitch over the years.