Dublin was red-carded for allegedly aiming a headbutt at Mills, but Grant felt that the former England full-back should have been dismissed himself for earlier elbowing Darren Huckerby.
"Dion has got sent off and I lose him for three games after he played ever so well, but Mills should have been off before that," fumed Grant.
"He's drawn his elbow at Darren Huckerby. I don't care whether he hit him or not, it's intent.
"I'm sure if Dion had headbutted Mills he would have been lying on his back."