Luciano Becchio scored the only goal of the game from the penalty spot in the third minute to end the Foxes' five match unbeaten run.
"We lacked quality and did not play well, the first half in particular," Pearson said.
"You'll hear managers in this division talk about consistency and we were quite a way short of what is our best tonight.
"It was never going to be easy to come here, but I have no complaints. The nature of this league is that the sides which achieve consistency tend to finish in a strong position. We have done that for the most part, but not tonight.
"We have acquitted ourselves ever so well this season. We have had games where we have not played so well, but tonight we put ourselves on the back foot early on."