Boykin was sent in when starter Casey Pachall suffered an injury to his left forearm in the second quarter with the score tied 14-14. Pachall did not return.
Boykin led TCU (1-1) to a go-ahead field goal in his first drive and the Horned Frogs then outscored Southern Louisiana 21-3 in the second half.
Pachall threw for 100 yards before leaving the game. Waymon James and B.J. Catalon both scored rushing touchdowns for the Horned Frogs.
Bryan Bennett accounted for 264 yards and two touchdowns for Southern Louisiana -- 132 yards and a touchdown passing and 132 yards and a score on the ground.