The Foxes were well below their best in front of their own supporters as they suffered a first loss in 21 Championship games just a few days after promotion to the Premier League was secured.
"It was a very poor performance," Pearson told reporters. "Unfortunately tonight we had too many individuals who under performed and collectively we have been very poor.
"The goals we have conceded were very uncharacteristic of us. They were poor goals. But we looked a shadow of what we have been about this season.
"So it was really disappointing to put on that sort of performance on what should have been an evening where we were celebrating getting promotion. So that's a big wake up for us."
Leicester return to action against Reading next Monday.