He said: "I believe it is difficult mentally for a player once he has gone into the top team to be completely focused in the Under 21s.
"I had that with Thierry Henry and Nicolas Anelka, when they went from the top team to the Under 21s in France and it was difficult. Sometimes people can feel they are rejected if they have to step backwards, then they do not completely perform.
"If England's first team play at the same time as the Under 21s then I don't think it will work for him to go with the Under 21s."
However, he did add: "But Theo would be happy to do it and I would not stand in his way."