The Scottish international was shown his marching orders by referee Willie Collum for a late and high lunge on home midfielder Scott Robinson.
Though admitting it was "out of character" from the Parkhead ace, ex-Gers star Boyd was in no doubt that the official made the right decision in showing a straight red.
Boyd said on Sky Sports: "It's horrendous.
"Scott Robinson is lucky not to come off the field with a serious injury. It's a horrendous tackle."