The 23-year-old right-back moved to The Stadium of Light in a £2million deal after growing frustrated at his lack of outings for Chris Coleman’s improving Welsh team.
Matthews made his senior Wales debut in May 2011 – against Scotland, in the Nations Cup – and has gone on to win 12 caps. His last appearance for his country came in a 2-1 World Cup qualifying defeat in Macedonia two years ago.
Matthews said: “The international thing was a major part of me moving.
“The league in Scotland isn’t great and I think the Wales manager saw that and hopefully this move will help me improve my chances of getting into the Welsh team because they are doing really well at the minute. But I just needed a new challenge.
“I have been at Celtic for four years and enjoyed every minute of it but I spoke to my agent and it seemed to be a good move for me at this stage. I’m happy to move forward and playing in the English Premier League was a big factor in my decision – Sunderland are a massive club, there is no doubt about it.
“It was a hard decision, I had grown close to a lot of the boys at Celtic but I just felt I needed something new. I think I have made the right decision. Celtic are a great club and I wish them all the best for the future.”
The full-back moved to Celtic from Cardiff City as a teenager and played almost 150 games in a four-year spell at the Glasgow giants.
He has become used to winning things – and now hopes that mentality will help his new club ensure they don’t become involved in yet another relegation tussle.
He said: “I will be going into this team with the same mentality that I had at Celtic. The manager has spoken and said he is hoping to bring in a couple more players so hopefully we won’t be in the relegation fight against this season.
“With the players they have here, they shouldn’t be in a relegation fight anyway, but I spoke to Dick Advocaat and he said he is going to try and get more players in which can only strengthen the squad.
“He seems really energetic and into his football so hopefully I can do a job for him because he seems like a great manager and a great man.
“Training has been brilliant and all the boys have made me welcome. The facilities are top class and I have settled in well already.
“I have only been here for a couple of days, and trained for one, but it feels as though I have been here for a lot longer. I thought it would be daunting but the boys have really helped me settle in.”
Matthews’ former team-mate Virgil Van Dijk has attracted strong interest from a number of Premier League clubs over the last two transfer windows – and the Welshman is not surprised.
He reckons the Dutchman will have a big decision to make if Celtic agree to do business with one of his south-of-the-border suitors.
He said: “There has been talk of him moving for the last couple of season’s but I think he is enjoying it up there like I did so I think the it’ll be a hard decision if the right people come in for him because he will have to think about it.
“He is a brilliant player for his age and for the way he plays you would think he is very experienced but he is only 23 or 24.
“He is still relatively young for a centre-back but he has a bit of everything – he has pace and he is good on the ball, so he will fit in to the Premier League very well if that ever happens. But I’m pretty sure he is still enjoying himself up at Celtic.”