He's always worked hard tracking back. His main weakness in the past has been his weakness. He seems to be filling out a bit though and his championship performances are promising that he can handle himself ok up against stronger, more physically players.
They can't knock him off the ball if they can't catch him. Plenty of great players on the slim side.
I'd have said, prior to this season where I've seen little of him, his main weakness was a lack of ruthlessness and aggression to take on players around the goal and look to score. But in this regard he did great on the summer tour and it sounds like he's been scoring and assisting pretty well on his loans.
He's a long shot to even make the squad, really, and not many new managers will come in and want to play a ton of reserve players.
However, we were really lacking in threat from wide positions last season, so the door is open. Januzaj was our best attacking player out wide, it wouldn't take much to outstrip Valencia, Nani or Young. If any of them are still here next season.