Arsenal manager Arsene Wenger admitted the injury sustained by midfielder Denilson against Everton was "mysterious".
The Brazilian, who had only just returned from a back problem, slumped to the ground unchallenged during the closing stages of Saturday`s 2-2 draw and left the field on a stretcher.
Wenger said: "It is a little bit mysterious at the moment. It could be an intercostal (muscle) problem. I've just seen him in the dressing room and he can walk quite normally. We have to see if it is a broken rib, if it is a deeper problem with his lungs."
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