Australian middle order batsman Adam Voges, suffered a suspected concussion at the Rose Bowl after being hit on the head by a throw while fielding against Hampshire.
The 36 year-old Middlesex skipper was helped from the field by both physios and will take no further part in the match.
He was struck on the back of the head by a ball thrown by the substitute fielder Ollie Rayner and missed by the wicketkeeper, John Simpson.
The Australian fell straight to the floor before being taken to hospital. He was released hours later, reportedly suffering headaches.
