"I spoke to him in the week and he said he didn't feel he could play today.
"He's still contracted to the club, he hasn't put in a transfer request and there are no deals lined up. But he needs to get his head sorted."
Lambert added: "He's been terrific for me - he's a brilliant goalscorer and I respect his decision, but we felt it was in the club's best interests if he didn't play.
"He needs time to get back into the right frame of mind, but this doesn't mean he won't play for the club again."