The 24-year-old was dismissed for an ugly lunge at defender Adam Chicksen after losing control of the ball during the first half of the 2-0 Capital One Cup third-round victory at MK Dons.
Gardner told the Sunderland Echo: "I feel sorry for Catts because he's been brilliant all season. His attitude in training and on the pitch is spot on. He's a true leader and I just feel sorry for him.
"He's the last person in the world who wants to get sent off. He was rock-bottom in the dressing room afterwards and everyone was trying to pick him up.
"Hopefully we will get him back in the team. From where I was standing at right-back, it didn't look too bad. But I could be wrong. He was the other side of the pitch.
"We have got to get behind him and lift him up."