Robson, who notched 90 England caps, said: "I'm surprised he's been left out.
"If it was my shout I'd have picked him.
"Then it gets all the media speculation out of the way before the World Cup qualifiers.
"The closer it is to those games the more pressure there is to give the 100th cap."
Meanwhile, another former United captain, Steve Bruce, is confident LA Galaxy midfielder Beckham will remain on Capello's radar.
The Wigan Athletic boss said: "I have known David for a lot of years and he will be devastated by this. But one of the things you have to admire about the kid is his resilience. This won't be the end of him as an England player, I'm sure of that.
"I believe he deserves that 100th cap. And when it does come along, he will regard it as his highest accolade in the game.
"He won't give up on it, he will come back."