Harrison, who nurtured him at youth level before his early promotion to Sir Alex Ferguson's senior side in 1991 and also coached him at international level when he was Welsh assistant to Mark Hughes, told the Manchester Evening News: "I was overjoyed when I heard the news that Ryan had signed his extension. No player deserves it more than him.
"I used to keep quiet but now if I hear anyone having a go at him, I say to them, 'Right, you've got £20m go out and buy me a new Ryan Giggs.'
"That's my stock answer because there is no way they could do it for that money. In fact there isn't another Ryan out there.
Messi has been dubbed the new Maradona and even Diego himself believes he's his natural successor as the King of Argentina.
"That's the kind of standard of player we are talking about who is the nearest to what Ryan Giggs has provided for United and still provides for them," added Harrison.
"That's how good he has been and still is. You wouldn't get Messi for £20m."