Welsh international Matthews signs for Sunderland

The 23-year-old -- who follows Uruguayan international defender Sebastian Coates in joining the Black Cats this week -- enjoyed a successful spell with Celtic winning four Premiership titles.

"We know that we need quality players to improve competition for places in the side, so Adam is a welcome addition to our squad and we are very pleased to welcome him to Sunderland," Advocaat told the Sunderland website.

Matthews, who joined Celtic from Cardiff in 2011, had been linked to several other clubs but Advocaat, who has stayed on in the post despite earlier announcing he was stepping down having saved them from relegation last season, won the battle to sign him.

"Adam is a player we have been tracking for some time," said Sunderland sporting director Lee Congleton said.

"Even though he is only 23 years old, he has a wealth of experience, and I am sure he will continue to improve under the leadership of Dick Advocaat."

Advocaat's recruiting of fresh faces is unlikely to stop there as the club has been linked with Zenit St Petersburg's Belgian international defender Nicolas Lombaerts and Dutch striker Jeremain Lens, who plays for Ukrainian outfit Dynamo Kiev.