RB Jamal Lewis suffered a serious concussion Sunday in Cincinnati and was visiting a specialist on Wednesday, according to a source with knowledge of the situation.
The Browns have made no comment on Lewis' absence.
The injury may force Lewis into retirement five weeks before he anticipated. He has said this would be his last NFL season.
Lewis may speak on the matter on Thursday after he receives more information on the extent of his injury.
The source said that Lewis was shaken up by the results of an MRI (magnetic resonance imaging test) that showed "abnormalities" of his brain.
Browns coach Eric Mangini made no mention of Lewis' injury in his daily press briefing.