Tbh I think he'll do really well at Juve. He'll guarantee loads of goals in an actual working system, Ronaldo always works better with a #9 to occupy the CBs, while Lukaku does have a hard working mentality (in terms of training and improving himself). He hasnt worked out here and I'm far from a fan, and he is annoying on the pitch, but he has shown plenty of times he can be hard working off the ball, it's more his role at Juve would be far different than his role at United. Lukaku will also look up to Ronaldo and work his ass off to impress him and work well with him, so I think itll work out for them.
If a big team is relying on him to be the main focal point of the attack and do things on his own while he's isolated like how we used him, then he won't be able to live up to the expectations IMO. Being part of the cast around Ronaldo, think he'd work fine.