England pace bowler James Anderson was forced off the field on day two of the third Ashes Test against Australia at Edgbaston with a side strain.
Anderson visibly winced after two balls of his ninth over, bowled one more delivery and then left the field, with spinner Joe Root finishing his over.
An England and Wales Cricket Board statement read: "James Anderson has a tight side and will be assessed overnight and in the morning.
"There will be an update tomorrow."
Although England are in a commanding position at Edgbaston, the problem could make Anderson doubtful for the next Test, which begins at Trent Bridge on Thursday 6 August.
Lancashire's Anderson - celebrating his 33rd birthday and playing his 37th successive Test - had figures of 1-15 from 8.3 overs in Australia's second innings before the injury to his left side.
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BBC cricket correspondent Jonathan Agnew, an ex-England seamer himself, explained: "They can take a while, these injuries.
"England will be desperate that their main man is fit for the next match. He is absolutely right to stop and go off."
Former Essex and England swing bowler Mark Ilott told BBC Essex: "It looks a bad one. If it's an intercostal it could be seven weeks [for him to recover]. Other than a stress fracture of the back there's not much worse than a side strain for a quick bowler.
"Potentially he could be out of the Ashes. When it swings there's no one better than Jimmy so I don't know how you'd replace him if it's as bad as it looked, but let's hope it's not."