The 24-year-old winger joined the Bundesliga side from Ajax Cape Town in 2018 and spent time in the club's under 19 and reserve sides ahead of a spell in Sweden and ultimate return to South Africa.
Sundowns take on the Bundesliga giants next Sunday, but first take on South Korea's Ulsan HD in their tournament opener.
They finish their participation in Group A with a tricky test against Brazilian Serie A side Fluminense.
Speaking ahead of departing to the United States for the competition, Matthews said he is looking forward to the upcoming games, as a way to get over the disappointment of losing in,the CAF Champions League final recently.
"I'm very excited, it's always exciting to play against a former team that I've played for and we know it will be difficult," he said in the Citizen.
"If you look at all the teams from around the world, I would say it'll be difficult for any team."
Matthews added though that he doesn't know a lot of the current Dortmund team. "Most of them are on loan. Some of them are transferred to other teams but I think there's one or two that are still with the team. The guys that work there, some I'm still in contact with," he explained.
“It was obviously a disappointment and the defeat (to Pyramids FC) was a very hard one. A few days after it's still the worst but at the end of the day we have another competition that we're in and we are going to do our best.”
Percy Tau has confirmed that he is open to offers from all PSL clubs - not just the so-called "big three" of Kaizer Chiefs, Orlando Pirates, or his former side, Mamelodi Sundowns.
Manchester City have confirmed their 27-man squad for the upcoming FIFA Club World Cup, with Jack Grealish a notable omission.
Chelsea have announced their 28-man squad for the upcoming FIFA Club World Cup, with new signings Liam Delap, Mamadou Sarr, and Dario Essugo included.
Borussia Dortmund have completed the signing of Sunderland midfielder Jobe Bellingham, five years after bringing his older brother Jude to the Bundesliga.
