The Welsh right-back said: “He just said ‘we’ve been watching you for a while, I like what I see but you’ve got to work on a few things and hopefully I can make you a better player’. I like to attack but the manager has already told me I need to work on my defending so I’ll try my best to do that.”
Matthews, 23, arrives at the Stadium of Light with a reputation as something of an attacking full-back, although defensive freedom during his time in Scotland would have allowed that.
Aware that opposing teams are not going to pose a great threat to Celtic, Matthews and his fellow full-back - usually Emilio Izaguirre - are given try creative freedom to attack at will.
In 150 games for the club, he netted five goals and registered an impressive 22 assists, but it is likely those tallies will lower whilst with the Black Cats.
Advocaat is keen on attacking full-backs, but the Premier League is a much tougher league than the Scottish Premiership, and Matthews' first task will be to defend. He has areas in which he needs to improve, and the Dutch coach made it clear to Matthews that he must improve his defending before becoming an automatic first choice in matchday selections.
For £2 million, Matthews looks like a wise signing by Sunderland, fitting into Advocaat’s preferred system and holding homegrown status. But it is clear he is still an unfinished prospect, and the 2015/16 season will be one of learning for the 23-year-old.