Sam Allardyce has told Jeremain Lens that he must work harder if he is to regain his place in Sunderland's starting XI.

The Dutch attacker joined the Black Cats in July from Dynamo Kiev on a four-year contract, but the 27-year-old has been struggling to make an impact for Allardyce's side.

Lens has failed to make the first team in recent weeks, and Allardyce says that the winger will need to work harder to break back into the starting XI.

"I think what Jeremain has to do overall, like they all have to do, is to lift his level up in terms of work-rate," Allardyce told the Sunderland Echo.

"There is no doubt at this level of football the size of the work-rate needed.

"With each position, I have the levels of an elite performer and an average performer for a Premier League player. It's easy for me to monitor."

Sunderland take on Crystal Palace at Selhurst Park on Monday evening.