Well I'm going to cause controversy here.
If we had that goal allowed for us - and I think we've had one or two this season - I would argue that once the ball came off the post it enters a new phase of play. The player would be offside if he had interfered with Kagawa's shot, but once that hits the post and comes back in the other direction it's first come first served.
Is that not the rules now, I think it is? They are different and a lot of people are still arguing for the old 'gaining an advantage' stuff. But then in truth does anyone really know, linesmen, referees, players, us?
I dont think so as the law is so fudged officials could easily uphold and justify either way.
That said, they do get a lot of dodgy stuff of course!!