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 saw Teddy playing recently at Burnley and he was the best player on the pitch.
"He did well at top level playing at the age he did but I don't think Ryan will play much beyond around 38. Teddy is a different player. He is economical and doesn't rely on pace or speed.
"Ryan's workrate is phenomenal even now and still has great pace but he isn't the type who'd accept playing at 50mph when he wants to be at 70mph.
"I don't think Ryan would accept that. He is like Eric Cantona in that he wants to be top class every match and gets really annoyed with himself when he doesn't perform to that level.
"I remember Eric in his last season before he retired how angry he was getting that his legs wouldn't do anymore what his brain wanted them to do.
"Ryan isn't at that stage yet by any means. He's got a few years left. He's still unbelievably fit and still gets fans off their seats.
"He's adapted his game and I believe his best role now is playing behind the front man. He played for Wales in that position for Mark Hughes and myself. He was behind Craig Bellamy and it was a devastating combination."