The Foxes took the lead in the Hillsborough clash, but a first-half brace from Connor Wickham gave the home side the lead.
The manager told the team's Twitter account: "Conceded two poor goals after good start, didn't work hard enough first half. Quality let us down, didn't take chances, frustrating night.
"Performance not of the standard we've produced during vast majority of the season. Can't have many nights like that with our aspirations."
Despite the defeat, Leicester remain at the top of the Championship table.