David Moyes has questioned the injury which has seen Lamine Kone rule himself out of Saturday's Wear-Tees derby as the defender tries to force through a move to Everton.
The Ivorian reported to the Academy of Light on Friday and told Moyes he had a back problem which meant he could not play against Middlesbrough.
This came just 24 hours after Kone rejected the offer of a new and improved contract at Sunderland.
Everton’s offer of £14million remains on the table but Moyes is ready to get tough with a player who only signed a four-year deal in January.