Giggs celebrated his 40th birthday yesterday.
"Ryan is obviously a different player now compared to when he broke through as a 17-year-old winger, but that says a lot about how football has developed in terms of sports science to make sure that players still have enough power and energy to play the physical part of the game," Berg told ManUtd.com.
"To be fair, last season he looked fitter than he did two or three years ago. That is a great achievement. He has a lot of help at the Aon Training Complex, but the biggest reason is his own mind, making the right decisions on and off the pitch. He has been fantastic.
"The way that football has developed, the central midfielder doesn't have to run from box to box or more than other players.
"Who is to say Ryan cannot play for another couple of years, or even three years? We just don't know, you shouldn't rule it out. For him to play at this level for so long has been unbelievable and I think it is still possible for him to go on."
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