A University of Arizona defensive assistant is going to wake up with a splitting headache Sunday ... 'cause he headbutted a player wearing a helmet out of excitement -- and cut his forehead wide-open.
Chuck Cecil -- a senior defensive assistant with the Wildcats -- was hyping up one of his guys pregame ... grabbing cornerback Owen Goss' helmet and slamming his head hard into the player's.
Goss was wearing a helmet ... but Cecil wasn't -- and, after smashing his head into the player's three times, he was clearly bleeding.