The result came after Sunderland claimed only their fourth win of the season on Saturday against Nottingham Forest, and the Barnsley defeat was the worst possible response, according to Coleman.
"It's a tough one to swallow after the game before at Forest on Saturday," the Welshman told the Sunderland Echo. "We felt positive going into it.
"Barnsley deserved it from the first minutes – they set the tempo a lot, were more aggressive and we looked nervous again at home. Disappointing.Even worse was the nature of the defeat.
"Even worse was the nature of the defeat. It is not a great way to start the New Year. We got zero and deserved zero. The goal we conceded was criminal. "For someone to take a corner and let it bounce in the box, it was unforgivable from us to let that happen."
Sunderland sit 23rd in the Championship table after the result, but one win could lift them out of the dropzone.