Allen, who scored 26 points in the second half on 9-of-13 shooting, wasn't about to get caught up in the excitement of his scoring.
"Every time I leave here I feel like I've left something out there on the table," he said. "I feel that way right now. There were some shots I should have knocked down."
The 12-year veteran had Pierce enjoying his work, though.
"It was great to watch. When you've got a hot player like that who's done it so many times throughout his NBA career, you just try to ride the hot hand," Pierce said "We just tried to get him the ball as much as we could. He made plays off the ball, on the ball and finished it with the shot."
The Atlantic Division-leading Celtics, who had lost three of their last four games and two straight at home, also got 12 points from Pierce and 10 from Eddie House. Boston had lost its last two games, both to Washington.
Portland, which lost for just the third time in 21 games overall, fell to just 6-12 on the road.
The surprising Trail Blazers may have come into the game hoping to compete with Boston instead of working like they have so far this season.
"I don't think we believed we could beat them," Roy said. "We've been playing to win lately, but tonight I think we just came in to give them a good game. The next time we play them we have to believe that we can actually beat them."
Allen scored 12 points, including eight straight in the third quarter, helping the Celtics take a 74-69 lead after three.
Portland tied it at 56 before Kendrick Perkins' free throw moved the Celtics ahead for good. House followed with a 3 from the right corner before Allen took over, scoring off drives to the basket three times and hitting a pair of free throws to give Boston 68-63 lead.
After the Trail Blazers cut it to 71-69, Allen completed his quarter with a reverse layup. He then opened the fourth with a jumper from the right baseline on Boston's first possession and Tony Allen had a breakaway dunk, giving the Celtics a 78-69 advantage.
Portland cut it to 80-75 on Outlaw's 3 with 7:55 to go, but Ray Allen hit a key 3, followed that with a turnaround jumper and added a 3 with 50 seconds left to help Boston seal the win.
Ray Allen capped his second half with four free throws in the final 31 seconds.
"They showed why they're the best team record-wise in the league right now," Portland coach Nate McMillan said.
Portland led 45-43 at halftime.
Boston PG Rajon Rondo was sidelined because of a sore lower back. ... Portland's Greg Oden, the first overall selection in last June's draft who was sidelined after season-ending right knee surgery on Sept. 13, spoke about the possibility of being a Celtic before the game. "I was on a plane (when the lottery order was decided in April) and it was like Boston or Memphis. Then, when I got off the plane, it was Portland," he said. Boston, which finished with the second-worst record last season, got the No. 5 pick and traded it on draft night to Seattle in a deal for Ray Allen. ... Oden said he was excited to see Garnett play in person. "Just being able to see what he does in real life," he said. ... Celtics reserve F Leon Powe was not with the team after his significant other, Lloren Cook, gave birth to a boy earlier in the day.
Kings snap 9-game losing streak, beat Mavericks 121-107
— De’Aaron Fox scored 30 points, Harrison Barnes added 24 and Terence Davis II 23 as the Sacramento Kings snapped a nine-game losing streak with a 121-107 victory over the Dallas Mavericks on Sunday night.
Fox stops and pops a nice jumper
De'Aaron Fox finds space and takes advantage with a jumper.
Rockets rally past Magic 114-110, snap 5-game losing streak
— Christian Wood scored 25 points, Kelly Olynyk added 24 and the Houston Rockets used a strong offensive showing in the third quarter to rally past the Orlando Magic 114-110 on Sunday night.