Matthews missed the previous two games with an undisclosed injury. But he took part in morning skate at TD Garden with his teammates and was on Toronto's first line for the matchup of the top two teams in the Atlantic Division.
Matthews netted his 21st goal of the season in the third period, drawing the Maple Leafs even with the Bruins, but Boston defenseman Matt Grzelcyk scored just his second goal of the season at the 18:44 mark of the frame, securing the win for the home team.
Grzelcyk slapped the puck through a screen and into the net. Toronto goaltender Matt Murray complained that he had been interfered with, but there was no challenge as the Maple Leafs fell to 3-4 in their past seven games.
Toronto went 1-1 in the stretch without their top player, but the offense clearly lagged, as the Maple Leafs amassed just three goals. Matthews helped the club match that output against the top team in the NHL, but it wasn't enough.
Prior to the game, Matthews described his injury as "nagging," and during his absence, Toronto coach Sheldon Keefe said the ailment has "been bothering him for a while."
"It's just something I've got to stay on top of with the medical team and the staff," Matthews said, "and making sure I get the care and treatment that I obviously need to perform every night."
Matthews -- who skated on his own Friday, an official day off for the club as he plotted his return -- now has 48 points this season, a year after winning the Hart Trophy as the league's most valuable player. In the loss, he finished with 27 shifts, covering 20:08 time on the ice.
"Obviously, you'd always like to be better and push myself to do more," Matthews said. "I have high expectations for myself. But at the same time, I think the way we've been playing as a team this year, defensively especially, I think that's just spoken to our growth as a team and our maturity and the way we've been able to win games."
Given the loss, the Maple Leafs, at 59 points, are 11 behind first-place Boston. But for the home team, the victory was especially sweet because it prevented the first two-game losing streak of the season.
"The game meant more to us than I imagined before the game," Boston coach Jim Montgomery said. "And I'm glad it did. We're proud of not having lost two in a row. There was a purpose to what we were doing -- not only because it was a second-place team in Toronto that we were playing, but we don't want to lose two in a row."
After the win, the Bruins continued the positive vibes, signing veteran forward Pavel Zacha to a four-year extension with an annual cap hit of $4.75 million.
Zacha, 25, has five goals and 20 assists this season, largely playing on Boston's second line. He had 22 shifts and 15:32 time on the ice Saturday.
Zacha, a 2015 first-round pick of the New Jersey Devils, was traded to Boston last summer for veteran forward Erik Haula. Zacha has 74 goals and 130 assists in his career after being selected No. 6 overall.
Haula, who scored in the Devils' game against the Los Angeles Kings later Saturday night, has four tallies with his new club.
The Associated Press contributed to this report.