Advocaat doesn't deny the winger's attacking ability, but says his work going the other way needs to be enhanced.
As reported by the Shields Gazette, Advocaat has called on Lens to up his defensive work rate and sacrifice, even if he does concede that the 27-year-old provides his team's most potent attacking threat at present.
“We needed a player (Borini) there that tracks back because that makes it easier for Patrick (van Aanholt)” said Advocaat. "And Lens has to improve that too. He must do that, otherwise it brings the team problems.
“He knows he still has to improve, but on the other hand, he is also the player with attacking threat in his game - more than the others.”
Lens has started all five of Sunderland's early season contests, and has proven himself to be one of the better players on show in a lacklustre start for the club.
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The Dutchman signed for Sunderland in July, for a fee of £8 million according to the BBC. His move to the North East signals his third spell working together with Advocaat; who he played under at PSV Eindhoven and AZ Alkmaar previously.
His newest venture with Advocaat has not got off to as great a start this time, however, as Sunderland sit second from bottom in the Premier League following a winless opening to the campaign.
They will have the chance to end that streak away at Bournemouth on Saturday; a team who have found top-flight life rather tough themselves in the season's opening five fixtures.