I prefer him celebrating than doing an obviously fake "non-celebration" like Ronaldo did. It's been clear since the weekend of Moyes' last game for us against Everton when all the papers had been briefed about Welbeck wanting to leave, that he didn't have any real respect for us, so why pretend now? It must've been a dream come true for him to score against us at Old Trafford and knock-us out of the cup - he'll probably be walking on air right now. Hell, he even got a largely good reception going off.
For some reason some of our youth products don't have any loyalty or appreciation to us as the club that made them - not just Welbeck, but also Cleverley turning-down a move last summer so he'd get a larger fee moving on a free instead of us getting anything for him, or Fryers going along with that Belgium scheme en route to Spurs so we'd get less for him on tribunal. Pogba of course needs no explanation. Even Cathcart and Eckersley I think left against Sir Alex's wishes IIRC. We can't assume loyalty from players just because we raised them - many of them will look after number one, and you can't really blame them.
At least with Welbeck, we've only got ourselves to blame. We shouldn't have sold him. We certainly shouldn't have sold him to a top 4 rival. And we certainly shouldn't have sold him so cheap. We, as a club, brought this on ourselves.
And given his age, we've got potentially many more years of this ahead of us.