Matsos was appointed to the head coach role only last month after working as an associate coach at the Ontario-based club last season.
His OHL team were playing on the road at the Barrie Colts when he took ill. The 44-year-old was rushed to hospital and the game abandoned.
Ina statement released on Sunday afternoon, the Bulldogs, said Matsos was “comfortable” and undergoing further tests.
“We would like fans to know that Dave is resting comfortably at Royal Victoria Hospital in Barrie where he will be undergoing tests today. He is doing well and in good spirits.
“We want to thank Barrie EMS, the Barrie Colts and their medical staff, the OHL and the hospital’s doctors and nurses. We also want to acknowledge the outstanding support of our Head Athletic Trainer, JP Laciak who was with Dave on the bench when he became ill.
“We thank everyone who has reached out in support of Dave and his family with thoughts and well wishes. We will issue another update when more information is available.”
Matsos coached the Steelers from 2006-2010 during which time he won two play-off titles, one of which came in a double-winning year with the regular season championship in 2009.