In this case,
forward is referring to the position that he plays.
See for instance responsibilities of a forward
To discuss the missing article, imagine if it said instead:
Former Manchester United defender says his old club is 'mad' to sell Danny Welbeck to Arsenal and expects player to be a huge success at the Emirates.
It's still a bit awkward to leave out the "the". You really should have it, but often news headlines will be written this way to make them briefer. @StoneyB has pointed out that people have given this style of writing a name and a Wikipedia article: "headlinese"
It's a little odd here to be using it on such a long sentence. But I suppose if one is worried about attention span and word economy, the first sentence in an article might use the same techniques as one might use in the headline itself.
When I first read it, I knew what it meant. But I still thought it was a poor choice on the writer's part. I'm fully sympathetic if you might have read it as:
Former Manchester United defender says his old club is 'mad' to sell Danny Welbeck to Arsenal and expects the team to have a huge success at the Emirates going forward.