They have already conceded 11 in six games this campaign, including the four at Old Trafford at the weekend - three of which came from corners.
Simpson said the new interpretation of the rules could be contributing to the club's defensive frailty.
"Err, I have to be careful what I say," Simpson told British media.
"It is in your mind, yes, we have seen a few decisions this season. It is in your mind. It is something we have to learn to adapt to as players. They have changed the rules.
"The way it was last season, I thought we were very good at defending set-pieces and we made it tough for the opponents.
"You have been doing that for all your career and suddenly you have got to change it, but it's the same for everyone, it's something we have got to work on and look at."
Leicester, who are 12th in the league standings, host Southampton on Sunday.
(Reporting by Ian Rodricks in Bengaluru, editing by Nick Mulvenney)
