Chris Martin's 42nd-minute goal looked like it might give the Rams a first triumph on Leicester soil since a 3-0 success at Filbert Street in February 2002 - when this was a Premier League fixture - until the introduction of Anthony Knockaert.
The Foxes substitute equalised with a stunning 25-yard free-kick after 78 minutes and then had a hand in Danny Drinkwater's winning goal just three minutes later.
Pearson, who made eight changes to his starting line-up, hailed Knockaert's influence after his 63rd-minute introduction from the bench but accepted that his side had not been at their best on the night.
"We know he's a talented player," said Pearson.
"I can't remember too many shots he's had like that hitting the target, so well done to him.
"He still has a lot to learn but he's come on leaps and bounds in the 14 months we've had him.
"It's good to get through, that was the aim at the start of the night.
"The performance was a mixed bag, but that happens when you make so many changes. But I expected that lack of fluency a little, although I wanted us to show a little more impetus.
"Although we were 1-0 down I don't think Derby caused us too many problems, the two centre-halves did well."