The Arsenal midfielder limped off the field following a tackle from Carlos Gruezo during last night's disappointing 2-2 draw with Ecuador.
"It would be an enormous blow to lose him. He did ever so well and looked really, really sharp," said Roy Hodgson after the game.
"His knee buckled under the challenge as the player turned into him. At the moment, he doesn't know how to feel because he doesn't know how serious the injury is."