Auston Matthews was blunt in his assessment of the Toronto Maple Leafs 6-5 overtime loss to the Senators on Monday, which came after the Leafs led 5-1 late in the second period. 

"A couple of careless plays," Matthew explained. "They challenged us, and we didn't really respond.

"We definitely blew this one."

Matthews' 13th goal of the season, and second of the game, put the Leafs up 5-1 with less than a minute remaining in the second period. Nick Paul then sparked the Senators comeback, capitalizing on a Morgan Rielly turnover for a shorthanded goal with just nine seconds left in the frame. Ottawa would then score four more unanswered goals, including Evgenii Dadonov's overtime winner.

The loss dropped to Toronto to 11-3-2 on the season and 1-1-1 against the NHL-worst Senators (4-12-1).

"It's discouraging and definitely a step back for us," Maple Leafs head coach Sheldon Keefe said. "I thought we were just really careless and sloppy with the puck. It's something that's been creeping into our game. It wasn't everybody, but it was enough guys that we gave the game to them at a time where they really had nothing happening. They only got what we gave them."

The loss spoiled Joe Thornton's return to the lineup after missing close to a month with a fractured rib. Thornton had a goal and an assist in the loss, which marked the first of three straight games between the two teams. The series will continue Wednesday in Toronto.

"A new day tomorrow," Thornton said. "I'm sure we'll look at some film tomorrow, work on some stuff and be ready for Wednesday."