Rooney's future is still to be decided after a late season falling out with retired manager Sir Alex Ferguson.
"In my mind the grass is never greener. I think Wayne Rooney suits Manchester United down to the ground. He can look around but his question will be: 'where can I get better than the best club in the world?' People may argue that is Barcelona or Real Madrid but for Wayne that is United," Robson told the Daily Mail.
"I get on well with Wayne and chat now and again and I know he is a passionate boy who is passionate about the club. He is a great talent and would be a big loss if he left United. And if we did lose him it is difficult to see who is good enough to replace him but hopefully he and the club can work it out."