Former Wales and Celtic striker John Hartson believes a desire for first team football is behind Adam Matthews' move to Sunderland.
Celtic agreed to sell the Welsh defender for £2m after signing fellow right-back Saidy Janko from Manchester Utd.
Matthews, 23, signed a four-year deal with the Premier League Sunderland.
"It's a brave decision from [Matthews], but I'm sure it's all about getting regular football," said Hartson.
"I think if he's not promised that at Celtic and he's guaranteed to play more at Sunderland, then I can understand."
Matthews joined Celtic from Cardiff City in 2011 and made 148 appearances for the Glasgow club.
He played 42 times as Ronny Deila's side retained the Scottish Premiership title in 2014-15.
Matthews, who has 12 caps for his country, has also played on the right of midfield at times for Celtic.
Hartson, who played 146 times for the Parkhead side between 2001 and 2006, said: "It's never easy when you have to leave a big club, and they don't come much bigger than Celtic.
"It can be a wrench to leave because the adulation you receive everywhere you go in Glasgow.
"To play in front of those magnificent 60,000 fans every home game, the opportunity to play in Europe as well, Adam would have tasted that as well during his time at Celtic."
Hartson thinks international ambitions with Wales was also a factor for Matthews, who has not played for Chris Coleman's side since the 3-0 defeat against Serbia in September 2013.
"He's certainly got the talent, he's got the ability," said Hartson.
"It's easy me saying he's going to Sunderland for first team football, when he goes to Sunderland he's also going to have competition there to get into the first team and stay there.
"But certainly I think it's the fact he wants to play regularly and if you are playing regularly then of course that's going to help you at international level."
Celtic manager Ronny Deila revealed on Saturday that it was an easy decision to sell Matthews to Sunderland.
"One year left on his contract, a good offer, and he wanted to try something else," he said. "It was the right time to do it."