Leicester, who held Everton to a 2-2 draw on the opening day of the season last weekend, frustrated Jose Mourinho's side for long periods at Stamford Bridge.
David Nugent had a golden chance to put the visitors in front 10 minutes after the break, but Thibaut Courtois denied the striker, who had gone one-on-one with the Belgian.
After Diego Costa had netted his second goal in as many games, Hammond was forced from the field with severe cramp, shortly before Eden Hazard made the game safe for the hosts.
"He got severe cramp right the way through his leg which he's never experienced before," Pearson said.
"It was frustrating for him because the last two games he's played very well.
"We certainly missed his organisational skills and when he went off we lost a bit of our rhythm somewhat.
"It was probably the worst timing for that to happen but these things happen in football and you've got to get on with it."
Despite the defeat, Pearson took the positives after a promising display from the visitors, who were promoted as champions of the second tier last season.
"The good thing is we created chances throughout the game and made their goalkeeper work," he said.
"We wanted to be positive and I think for the most part we played very well.
"But as always it's going to be how efficient you are in both penalty boxed and we didn't quite find the right way today."
