Al Horford made his preseason debut and finished with 2 points and 2 rebounds in 23 minutes, making one of his six field goal attempts. Horford has been conditioning and practicing with the team, but Mazzulla wanted to limit his contact during the preseason.

“I saw he looked great, he recovered and was in the right place defensively,” Mazzulla said of Horford. “He communicated. He was solid.”

Mazzulla wanted to get Horford some extended minutes each week before the season starts. The Celtics are going to be cautious with the 38-year-old center, especially since his responsibility will increase with Kristaps Porzingis’ ankle injury. Mazzulla said he was impressed with Horford’s ability to “reinvent his game” during training camp.

“I felt good, it felt good just to be there, be part of a game, and get a feel for it,” Horford said. “I’m quite happy to be back. I think I’m in a good place (physically). The good thing is that we still have a week before we start the season, so I will continue to prepare and be in a better place.”

Jayson Tatum led Boston with 24 points, making five of his 13 3-point attempts, while Derrick White added 23 points on 7-for-20 shooting.

The Raptors, who were shut out Sunday at TD Garden before making a late rally with their second unit, exploited the Celtics’ transition defense with the speed of Gradey Dick, who led Toronto with 27 points.

“They are a fast team, well coached, fast in the half too,” Mazzulla said. “The transition game is the most important. It’s something we have to work on and that will be good for us.”

The Raptors shot 50 percent from the field and made 10 threes in the opening period, forcing the Celtics to rally most of the night. Boston responded with a 26–2 run to take a 1-point lead at halftime, but was erratic in the third and could never regain control.

Observations from the game:

⋅ It was a tough third quarter for the Celtics as they couldn’t contain Toronto’s big Jakob Poeltl, who scored off Horford in the post, added a midrange jumper and then tipped in as Boston’s defense slipped. Toronto made a habit of pushing the ball after Celtics created the ball and turning those transition opportunities into buckets. Dick sometimes enjoyed a celebration, going to the basket for layups after beating his defender to the ground. Tatum also had trouble defending All-Star forward Scottie Barnes, who nearly collected a triple-double with 26 points, 10 assists and 9 rebounds.

⋅ An encouraging sign remains the overall play of Payton Pritchard, who knocked down four of the Celtics’ 10 three-pointers in the first half and made 21 of his 44 attempts in the preseason. Luke Kornet also came off the bench in the second quarter after starting in place of Horford and was a plus-16 in his seven-plus minutes as the Celtics defeated Toronto 40-20.

⋅ Walsh was one of the first reserves Tuesday, perhaps an indication that the second-year swingman is carving out a role on the second unit. He has played with more poise and confidence during the preseason, adding a three-pointer in the flow of the attack. He finished with 7 points and three rebounds in 24 minutes. Tatum and Horford each pointed to Walsh as one of the most improved players in training camp.

⋅ The Raptors jumped the Celtics with accurate three-point shooting from Barnes, who scored four in the first 4½ minutes, and Dick, who added three in the first quarter. Boston was slow in transition defense and the Raptors attacked after making baskets, jumping out to a 44-20 lead with 2:20 left in the period. Toronto made some tough three-point attempts, but the Celtics were slow in coverage.

⋅ The Celtics have six days to prepare for next Tuesday’s season opener against the Knicks at TD Garden, but will take Wednesday and Thursday off before practicing Friday. The preseason is generally used to get players back into top form and iron out some early issues, but the Celtics steamrolled opponents through the first four games and had a dominant stretch on Tuesday.

⋅ The only starter to get a break in the preseason finale was 34-year-old Jrue Holiday, who did not make the trip.

Gary Washburn is a Globe columnist. He can be reached at [email protected]. Follow him @GwashburnGlobe.