"The Dutch international has joined the Black Cats on a four-year deal, for an undisclosed fee," the Premier League club said on Friday in a statement on their website (www.safc.com).
The 23-year-old struggled to break into Chelsea's first team after arriving at Stamford Bridge as a youth team player in 2007 and spent the last two seasons on loan at Dutch side Vitesse Arnhem.
Capped twice by the Netherlands, he was a member of Louis van Gaal’s provisional 30-man squad for the 2014 World Cup but did not make the final 23 for the tournament finals in Brazil.
He becomes the Black Cats' fourth close-season signing after the arrivals of defender Billy Jones from West Bromwich Albion, midfielder Jordi Gomez from Wigan Athletic and goalkeeper Costel Pantilimon from Premier League champions Manchester City.
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(Reporting By Tom Hayward; editing by Toby Davis)
