With Di Maria on the verge of joining Paris Saint-Germain after just one season at Old Trafford, Bale has once again been linked with a move to United.
Madrid boss Rafael Benitez insists the 26-year-old is not for sale, but Robson talked up a possible pursuit of the winger and pointed to his ability to play in more than one position.
"Bringing Bale in would be a really good signing if it was to happen," said the 58-year-old, speaking at the launch of ESPN's Greatest Sporting Cities research.
"The good thing with Bale, and this is what the manager likes, is he can play in two different positions - he can play wide coming in and also as a centre-forward.
"It will be really difficult but if the manager can add him to the squad I think he will be a happy man."