Parker believes Giggs will have "frightening" knowledge from all his years in the game which the club can use to its advantage.
The former England defender, 49, said: "Maybe this move goes beyond playing and is also about using his experience.
"As a player, Ryan is a threat - we all know he will be of great use in the squad - but he is also a threat with his knowledge of the Premier League.
"He will go on and on and that is a frightening thought for the rest of the Premier League. He simply has to be there - and for a very long time.
"Certain things happen where people come in and do it their own way. But the Manchester United way is proven, there is a method to how it works.