But the striker has been at his very best in the early weeks of the new season.
"I was quite worried when I came in because I had lost Rene [Meulensteen, a former United coach], who I knew was quite close to Robin," said Moyes. "But I've got to say ever since he joined up in pre-season in Sydney he has been terrific.
"The biggest thing for me is how much he has bought into everything we're doing and his work ethic.
"There was a moment in the game last weekend, probably around 75 minutes, when he worked back and did a job for the team. It shows how much a team player he is at the moment.
"I asked him the other day how many medals he has won and he said one. I saw the glint in his eyes to say 'I need more than one' - and so do I.
"His movement in the box is sensational, it really is top notch. There are some things that I don't think are even coached, they are nearly instinctive, which you pick up if you're a top player.
"I used to wonder how Ruud van Nistelrooy scored so many. I used to think he was an unbelievable goal scorer.
"Robin has a little bit of that, something different. He has an array of types of goals. He scores from outside the box, inside, he scored an unbelievable header in the Community Shield, he scored another on Saturday when he got his foot up near seven foot [high]. I can only say he has been brilliant to work with."
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