EMPLOYEE WAS INSTALLING A COVER FOR THE HEADGATE DRIVE SPROCKET IN THE E-13 LONGWALL SET-UP SECTION. THE EE WAS ASSISTED BY ANOTHER EE THAT WAS USING A PRY BAR TO HELP INSTALL THE COVER IN TO THE CORRECT POSITION. THE PRY BAR SLIPPED AND THE COVER CAUGHT THE EMPLOYEE'S LEFT THUMB TIP BETWEEN THE COVER AND THE DRIVE ASSEMBLY. DIAGNOSED AS A FRACTURE & SUTURES WERE REQUIRED.