Walcott hurt an ankle during training with England on Monday, needed to be helped from the pitch and could not complete the session.
Arsenal winger Walcott is now a doubt for Wednesday's international friendly against Scotland - and his club boss Arsene Wenger will be anxious no risks are taken ahead of the Gunners' season-opener against Aston Villa.
England coach Roy Hodgson said: "Theo took a slight knock at the end of training, but we believe he'll be okay. He's very much still in contention."
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