The former England centre told Sport Wales: "Personally, I just don't think Sam's ready to be captain of the Lions. He's proved beyond doubt that he can captain Wales, winning the Grand Slam.
"But if the rumours are true that he turned down the captaincy in the last campaign, then that puts a massive doubt over his selection as Lions captain. I think that would be dangerous.
"If the Welsh squad players know that he turned that down, and we're only speculating at the moment, then I think it would be even difficult for them to swallow."
The tour of Australia takes place during June and July.