The Celtics close out the preseason with a game against the Raptors

TORONTO – The Celtics finished this preseason healthy and played with the same dynamism and precision as last season despite a last-second loss to the Raptors on Tuesday night at Scotiabank Arena.

While the Celtics' first team had overwhelmed their opponents in the first four games and responded again with a rousing run in the second quarter, they were unable to continue the momentum in the second half as coach Joe Mazzulla finally scored his first unit midway through the game fourth quarter.

The third-unit players, rookies and training camp invitees came together for a late rally, but Jordan Walsh's last-second layup attempt was blocked at the rim and the Celtics suffered their first loss, 119-118.

Jayson Tatum led Boston with 24 points and made five of his 13 3-point attempts, while Derrick White added 23 points on 7-for-20 shooting. The Raptors, who were eliminated at TD Garden on Sunday before making a late surge with their second unit, took advantage of the Celtics' transition defense with the speed of Gradey Dick, who led Toronto with 27 points.

The Raptors shot 50 percent from the field and made 10 three-pointers 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 halftime lead, but was erratic in the third quarter and never led again.

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

Observations from the game:

⋅ It was a difficult third quarter for the Celtics as they couldn't contain Toronto star Jakob Poeltl, who found Horford in the post, added a mid-range shot and a tip-in as the Boston defense slipped. Toronto has made a habit of pushing the ball to the Celtics and turning those transition opportunities into buckets. Dick, the sophomore guard, was celebrating at times, going to the basket for layups after knocking his defender down.

⋅ An encouraging sign continues to be the overall play of Payton Pritchard, who fired four of the Celtics' 10 3-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 early in the preseason and was plus-16 in his seven-plus minutes as the Celtics outscored Toronto 40-20.

⋅ Walsh, the second-year swingman, was one of the first relievers to enter the game, perhaps an indication that he is carving out a role in the second unit. He played with more poise and confidence in the preseason and contributed a three-pointer to the offense. He finished the game with 7 points and three rebounds in 24 minutes.

⋅ The Raptors outpaced the Celtics with accurate 3-point shooting from Scottie Barnes (who scored four goals in the first 4½ minutes) and Dick, who scored three. Boston was slow in transition defense and Toronto attacked for baskets to take a 44-20 lead with 2:20 left in the first period. Toronto made some difficult 3-point attempts, but the Celtics were slow to cover.

⋅ Mazzulla wanted to give Horford some extended minutes a week before the season started. The Celtics will be cautious with the 38-year-old center, especially as his responsibilities will increase with Kristaps Porzingis' ankle injury. The coach said he was impressed with Horford's ability to “reinvent his game” during training camp.

⋅ The Celtics have six days to prepare for next Tuesday's season opener against the Knicks at TD Garden, but are taking Wednesday and Thursday off before practicing on Friday.

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

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