Of course we can't tell much from one match, but to do what he did shows a temprament that will hold him in good stead, and by scoring two on debut he's bought himself a few more opportunities to prove himself in the long term as well. That's the advantage he now has, had he started a few games without scoring, he might have found himself farmed out to a lower league team on loan. Now, even if he doesn't break straight into the Utd first team, he's got a much better chance of turning out for a mid table premier league team on loan.
That might be considered a double edged sword as he'll be developing in the public eye, but if you're good enough, you're old enough. All we can say now is that on debut he managed to score more goals for the season than Kagawa, Nani, Fellaini and Cleverley combined (poor old Fellaini did have a hand in both though!).
Moyes was given plaudits for giving Adnan Januzaj his opportunities, in fact it might be one of the few ways he'll be remembered positively by the Old Trafford faithful. Ryan Giggs may be remembered as a legend as a player, but also the man who took a punt on an 18 year old who took that opportunity with both hands.
As a little aside, did you know Ryan Giggs is actually Ryan Wilson? He does get about a bit, but it doesn't appear to be a relation!