There was some speculation, given the current climate around the NHL and especially in Ottawa, that the absences were COVID-19 related. However, there was no confirmation at the time on the reason for Matthews and Chynoweth being held out of practice, other than that the move was precautionary. We now have that confirmation.
Per multiple sources close to the team, Matthews and Chynoweth have both produced positive COVID-19 test results today, which is why they were held out of practice, proving the speculation to he correct.
However, due to the fact that the Leafs just played the Ottawa Senators on Saturday night and that the team is just moving past a massive outbreak in the locker room prior to the holiday break, it doesn't look good.
Thankfully, if Matthews and Chynoweth end up being asymptomatic, we could see both back within 5 days, as opposed to the original 10 days that they would have had to isolate, per the old regulations.