The 36 year-old already has four goals in three games for United this season.
Asked how long the veteran forward could go on, he said: "I wouldn't be surprised if he still played when he was 40, not at all, because of the way he looks after himself.
"That's the key to it - and of course some genes. There must be some genes as well, some DNA in there.
"He has put every single ounce of energy and effort into becoming the player he is and has been, so he deserves every single little plaudit that he gets for his physical state.
"What's more impressive is when you have achieved as much as he has, he's still as hungry. His mentality is still absolutely spot-on and that's a desire from inside that he's going to keep going until his head says, 'no, I've given everything now'.
"Hopefully it will last a few more years. Linford Christie (who won the Olympic 100m aged 32 in 1992) was quite an age when he won his gold as well, so age is never a problem."
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