Robson now marvels at the staying power of the player who broke Sir Bobby Charlton's Manchester United appearance record.
Robson told the Sunday Mirror: "Ryan will be 38 in November and for me there's no doubt he can go on for a few more years yet.
"He's got the desire - that's why he's won as many things as he has in his career.
"When I saw him as a 17-year-old when he first came in he had unbelievable pace, stamina, and that hasn't changed much.
"He won't put weight on and he's a fitness fanatic. He also has an incredible mentality to keep going and keep winning.
"He has more of a temper than you'd think. He's mild-mannered, but when he really thinks something, he can snap and have a right go at people.
"You need that edge. You have to care and have a bit of edge to be a top player."