The eight-time grand slam winner played for the last time in 1994 but since joining Murray has been ever-present in the player's box during the Australian Open's early rounds.
But he was hit by a broadside from Courier, a commentator for Australian TV, who said the Czech had financial incentives for his move.
"There is a reason and it is a little bit more mercenary," Courier told Channel Seven, according to Britain's Daily Telegraph.
"He hasn’t been allowed to make any money from tennis for the past 15 years because he cashed in disability insurance. That is the reason Ivan is back playing."
But Lendl called Courier's comments "ridiculous", according to the Telegraph.
"Jim shouldn’t be saying stuff like that... it’s wrong and he doesn’t have the proper information. End of story," he was quoted as saying.