The Wales international, 23, joined the Black Cats from Scottish champions Celtic last week and has revealed that his first week at his new club has whizzed by.
Matthews won four Scottish league titles in four years with the Hoops but is excited to now test himself in the Premier League with the Wearside outfit.
"The move happened quite quickly to be honest," right-back Matthews told the Chronicle.
"It hasn't really had time to sink in yet, but I am enjoying it.
"I enjoyed my four years at Celtic, it's a massive club and the fans were brilliant with me.