Boro took the lead in the 24th minute through full-back Justin Hoyte, who picked up a loose ball on the left and saw his cross beat everyone - including Steele in the Barnsley goal.
Gordon Strachan: "It was a fluke.
"We won the football match in the first half and they decided to turn it into a cup tie and then they won it.
"We were disappointed to lose the game but we couldn't match them in a different game.
"There was nothing wrong with the first goal and the second one was made by a good cross."