"It's tough to get worse from here," Matthews said, according to TSN's Kristen Shilton. "It's frustrating. In the locker room, confidence isn't exactly high right now. We have nobody to blame but ourselves. It's really up to us."
The Leafs found themselves trailing early once again after the Penguins opened up a 2-0 first-period lead against backup netminder Kasimir Kaskisuo, who was making his NHL debut.
Toronto has now allowed the opening goal in seven straight contests and has only two regulation wins in its last 15 outings.
"We're 22 games into the season," Matthews said. "These points start to matter. We can't just say, 'It's just one game. We'll get it next time.' It's been repetitive, and for us, we want to make sure it stops now and we want to get back on track."
With a 9-9-4 record, the Leafs have suffered their worst start through 22 games with this core group and sit two points out of a wild-card spot.
Toronto continues its six-game road trip with a battle against the Vegas Golden Knights on Tuesday.