However, even at 21-4 overall, with three straight wins, there's work to be done.
"That opening game here for us was very emotional, with all the Gordon (Hayward injury) stuff going on, after a tough loss in Cleveland, it was just a lot," Horford said after helping the Celtics to a 111-100 victory over the Bucks on Monday night.
"They got the best of us that night, and I felt like as a group we've grown tremendously in very little time and I feel like we have a lot more to do -- a lot more growing to do."
Kyrie Irving scored 32 points and the Celtics overcame 40 points by Giannis Antetokounmpo in the latest win.
Horford, playing another strong all-around game, fought off second-half foul trouble and finished with 20 points, nine rebounds and eight assists in the win.
During this new winning streak, which followed a loss to the Detroit Pistons in the opener of a five-game homestand, Horford is averaging 18.3 points, 7.3 rebounds and eight assists. Irving is averaging 29 points in the three victories.
"When you've got guys who are willing to be as selfless as we are, then it makes everything in terms of our system work," Irving said of he and Horford. "Me and Al, we're pretty selfless guys, but we understand when we've got to make our mark in the game and if we do that, whatever's needed we do."
Horford was a plus-21 in 34:40. He picked up his fourth foul with 7:32 left in the third quarter and never did get a fifth.
Horford became the first Celtic since Larry Bird in 1984 to produce 20-plus points, nine-plus rebounds, eight-plus assists and 80 percent-plus shooting from the floor (he was 8-for-10) in a game.
Irving and Horford combined for 36 points after halftime, allowing Boston to overcome another big night by the Greek Freak.
"I feel like I'm just stating the obvious about (Irving) and Giannis. They're good," Celtics coach Brad Stevens said.
Boston rookie Jayson Tatum hit four consecutive 3-pointers and scored 14 of his 17 in the first quarter.
"They were actually baiting him to shoot the ball and he just wasn't hesitating," Horford said. "It was just a lot of fun to see him have some success there early and I felt like because of those threes he kind of opened the game up for the rest of us."
The Bucks, who had one home game after a western trip and then came to Boston, were sluggish in the first half but had something left to make the game interesting after the break.
"It's tough to win here but they gave themselves in the fourth quarter an opportunity and we just couldn't get over the hump," Milwaukee coach Jason Kidd said. "The first half we were awful."
Said Antetokounmpo, "They just played harder than us, but at the end of the day, I thought our effort was good."
Aron Baynes had 10 points in the win, while Jaylen Brown got in early foul trouble but finished with eight points and seven rebounds in 22:58 for Boston.
Irving, who shot 13-for-24 from the floor, had 19 points in the second half, including five in a row after his team was up by seven with 90 seconds remaining. He also contributed four rebounds and a pair of steals.
The Celtics, who took the lead for good at 7-6 and led by as many as 20 points, have been on fire shooting the ball. This was their fifth straight game shooting better than 50 percent from the floor, and they have six in a row with at least 10 3-pointers while going better than 40 percent from behind the arc in those game.
They have won 11 of their past 12 at home and 21 of their past 23 games overall.
Boston ran up a whopping 45-28 rebounding advantage.
Antetokounmpo, delivering his third straight 30-point game, was 14-for-24 from the floor and 12-for-14 from the foul line. He also had nine rebounds, four assists and four turnovers. Most of his baskets came driving to the basket, his outside shot a struggle.
The Bucks fought back from a 20-point third-quarter deficit to get within seven. They could get no closer as the Celtics won the season series 2-1.
Milwaukee (12-10) lost for the second time in its past seven games.
NOTES: Boston G Kyrie Irving said he could shed the mask protecting a slight facial fracture as early as Saturday. ... The Celtics were the first team in the NBA to reach 20 wins, accomplishing the feat for the 15th time in their storied history. They reached the NBA Finals in 13 of the previous 14. ... It was Greek Heritage Night at TD Garden, fitting with F Giannis Antetokounmpo, the Greek Freak, in town. He received a huge ovation in pregame introductions. ... The Bucks play at Detroit and the Celtics host the Dallas Mavericks on Wednesday night.