Millwall went into the match having remained unbeaten through their opening three games of the season, but the London outfit were unable to prolong that streak at Hillsborough.
Harris was quick to criticise his players for their performance in Yorkshire, claiming that he was "not prepared to accept" the display of his side.
The 41-year-old is quoted by BBC Sport as saying: "The first goal was a great strike but the second was a joke of a goal and we gave ourselves a mountain to climb.
"Those standards won't be accepted and my words were quite scathing to the players. I let them know what I thought in no uncertain terms.
"I'm the angriest I've been for a long time. We're a good team but tonight we didn't show it and I'm not prepared to accept that."
Millwall are next in action when they travel to Rotherham United on Sunday.