"I can't understand why it was disallowed," McClaren said after the 2-0 defeat at St Andrew's.
"I've seen the replay and it looked a perfectly good goal. But that's no excuse for our defeat. Not every decision goes your way and our performance was very poor.
"We did not start the game with the intensity of Birmingham City and this was a good opportunity to get our Christmas programme off to a good start.
"We didn't do justice to our league position or the talk of us qualifying for the Champions League. If we want to achieve that, then we cannot afford more off-days like this."