"We just folded, kind of just quit," Matthews said postgame, according to TSN's Mark Masters.
Matthews added, "We can't be doing that, not to a guy like (Frederik Andersen), not to our starting goalie. Have to have a little more pride than that."
The Leafs allowed five third-period goals, with four coming in the final 3:28. The Flyers scored an empty-netter to extend their lead to 4-1 then, with Andersen back, added two more just 12 seconds apart to complete the blowout.
"We have to play to that last minute, because that's just unacceptable," Matthews said. "We just left him out to dry ... suddenly the score's 6-1 so that's on us. That just can't happen."
It was the Maple Leafs' worst showing under new head coach Sheldon Keefe, who denounced the way his team closed out the contest.
"Our response, our reaction after giving up the empty-net goal is not what we want to be about," Keefe said, per Masters.
"We want to be a team of high character and (a group) that cares for one another and we just left our goaltender completely out to dry there and stopped playing," Keefe added.
Toronto will look to bounce back Wednesday when they host Nazem Kadri and the Colorado Avalanche.