The Palace captain was out of contract at Selhurst Park, but despite being offered a new deal chose to sign a three-year contract with the Addicks.
Warnock, apparently stung by his captain's lack of loyalty, thinks the 26-year-old has made a mistake.
"I think the saying that comes to my mind is the grass is not always greener on the other side." Warnock told the Croydon Guardian.
"I think it's a mistake. He will be financially better off and you cannot blame him for that, that is what Bosmans are about, but sometimes it's not all about that.
"Obviously we are disappointed but I think we have done well for him. I don't think he would be going to Charlton now if I didn't come to this club.
"I thought he was going nowhere in the first few games I saw.
"I made him captain and it lifted his confidence and then he had a good season."