Allen scored 28 points Wednesday night to lead the Celtics to a 92-81 preseason victory against the Minnesota Timberwolves in Garnett's first game against his former team.
Garnett, who spent 12 years in Minnesota, scored nine points in less than 30 minutes at London's O2 Arena, but he helped in other ways.
"When you have KG in the post, everyone wants to double him," said Allen, who was 5-of-8 from 3-point range. "We can knock down 3s. We have so much available."
Garnett had six rebounds, two assists and three blocks.
"The past is the past," Garnett said. "This is a new chapter in my life."
It seems Ricky Davis hasn't forgotten his old teammate, however.
"It was definitely a different experience seeing Kevin in green," said Davis, who scored a team-high 18 points. "I almost threw him the ball once, but then I saw that green jersey."
The Celtics ran away from the Timberwolves late in the second quarter. A 15-6 run gave Boston a 50-43 lead at the half, and a 15-5 run to start the third pushed the lead to 65-48.
Garnett scored only two points during the stretch before halftime, but he set up Kendrick Perkins for two straight dunks and had a steal in between. Allen, Boston's other big offseason addition, had nine of the 15 points at the beginning of the third.
"Kevin's going to do what he's done for 12 years," Timberwolves coach Randy Wittman said. "Kevin's going to score, rebound, block shots. Kevin is going to be Kevin, and that's what he was tonight."
Boston's new "Big Three" of Garnett, Allen and Paul Pierce sat out the fourth quarter, and the Timberwolves went on an 8-0 run to cut the lead to 80-75 with 6:53 to go.
"We have three guys at the exact right time," Celtics coach Doc Rivers said. "They've done everything. They've achieved everything individually. Now they want to achieve something as a team."
Pierce, who led Boston in scoring last season, finished with only eight points and was 6-of-11 from the free throw line.
"That's the way it's going to be for us on any given night," Rivers added. "One, maybe two are going to get going. The night we get all three going, it's going to be a fun night."
Still, the combination of star players is expected to help the Celtics contend for its record 17th NBA title.
"Kevin and I are here now," Allen said. "We know what it's like trying to carry a team. We can help each other make this team better."
The atmosphere at the sold-out O2 looked familiar to some. The dancing girls, the mascots and the dunking team all made for a regular NBA atmosphere.
"I said it a couple of times on the bench, 'This is the NBA right here,' " Allen said.
And don't forget the celebrities at courtside.
Formula One driver Lewis Hamilton drew one of the biggest cheers of the night when he was shown on the video screen. Chelsea striker Didier Drogba also was in the crowd, and Celtics great Bill Russell also drew a huge cheer when he was introduced at center court during the second quarter along with Rick Barry and B.J. Armstrong.
"Seems only appropriate that there's an NBA team here," Allen said of the celebrities.
The Celtics and Timberwolves traded the lead throughout the first quarter and the start of the second. Davis scored 16 of Minnesota's 22 points in the first quarter, hitting four 3-pointers.
Garnett made his first field goal late in the first quarter, dunking from the baseline to give the Celtics an 18-17 lead. But he also got called for traveling several times, including two straight infractions that were followed by a bad miss from the field.
Wednesday's game was part of NBA Europe Live, which also includes the Toronto Raptors and the Memphis Grizzlies. The Celtics beat the Raptors 89-85 Saturday in Rome, and the Wolves beat Turkish team Efes Pilsen 84-81 in Istanbul.
"I'm pleased with what came out of Turkey and London, but we still have a lot of work to do," Wittman said.
Late last month, the NHL opened its regular season at the O2 Arena. The Stanley Cup champion Anaheim Ducks and the Los Angeles Kings split the series in the first regular-season games to be played in Europe. On Oct. 28, the NFL will play a regular-season game at the rebuilt Wembley Stadium when the Miami Dolphins face the New York Giants.