"I wouldn't want to say anything against David Beckham because he's a fine boy and has served our country well," Robson told Radio Five Live's Sportsweek programme.
"In the beginning, I would [pick him] to see how he goes. In the short term, he looks okay.
"But the longer he plays in America, with respect to the football being played there, the less competitive he will become.
"It is not the greatest level of football. The travel from Los Angeles to England and back again is tremendously difficult.
"He will lose the match sharpness he needs to play at the top level with England."
Robson also believes Beckham will find the gruelling schedule difficult, saying: "How he can play for England on a Wednesday after playing for Los Angeles on a Friday night, I will never know."