The Welsh midfielder limped out of the Upton Park triumph after the hour-mark with manager Arsene Wenger suggesting he may now miss some action for the league leaders.
"It looks like a thigh strain. For me, I don't know how serious the thigh strain is but the Christmas period is certainly over for him. It was his birthday today,â Wenger said.
"I don't know how long it will be. We will have to see tomorrow morning. He knew straight away, but we have to see."
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