Leicester conceded from three corners in the first half at Old Trafford – including two headers by Chris Smalling and Paul Pogba – and former Manchester United defender Simpson believes the new rules have had an effect on how they approach set-pieces.
"It is in your mind," said Simpson. "They have changed the rules, and it's something we have to learn to adapt to as players.
"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 all your career and suddenly you've got to change it. But it's the same for everyone.
"I'm sure we will look at it and work out the best way because that can't happen again."