also called for a review of the rules.
"It was a really, really complicated and difficult decision," he said.
"I think the decision they have made comes from the rules that there are in place in this moment and so maybe at the end of this season we can think if it's possible to change this type of rule but in this moment it's like this and we have to respect the rules."
Chelsea's defeat at West Ham ended a run of three successive Premier League wins and means victory against struggling Watford at Stamford Bridge is vital in the race for Champions League spots.
Chelsea, in fourth spot, are just two points ahead of Manchester United and Wolves.
Lampard challenged his players to restore the mentality that underpinned the good run that preceded Wednesday's lacklustre 3-2 loss.
"Every game now is going to be tough," said Lampard.
"If we go into it with the right attitude that we approached games like Aston Villa and Manchester City, and Everton and Liverpool before the break, then I have real belief in us.
"It's about getting our mentality right, keeping freshness in the players between games and having a real desire to achieve want we want."