Indeed, Crystal Palace man Ledley, who spent four years at Parkhead, took to social media to congratulate his 'brother,' who yesterday secured a switch to the Premier League side.
Matthews joins Sunderland after surprisingly quick negotiations that no doubt caught many supporters by surprise.
The defender enjoyed success with Celtic but is now looking forward to a new challenge with the Black Cats.
Crystal Palace and Wales midfielder Ledley, meanwhile, has enjoyed relatively more success than several others who have joined him in leaving the Bhoys for English clubs.
The 28-year-old has done far better than Gary Hooper, for instance, although Victor Wanyama and Fraser Forster have also enjoyed success.
Matthews' Sunderland move will now be an interesting case to witness. Dick Advocaat's defence certainly needs strengthening but is the ex-Celtic defender the man for the job? Ledley will no doubt relish a reunion with Matthews when Crystal Palace face Sunderland...
Celtic lost 5-3 to Dukla Prague on Saturday afternoon, having previously beaten Den Bosch 2-0 in pre-season.