And yet their highly-anticipated return to Boston Sunday night -- Game No. 42 of the regular season for the 2013-14 Brooklyn Nets -- proved to be the most difficult game either of the future Hall of Famers has ever had to endure.
"I was telling Kevin, I was telling everybody -- this was the toughest game I ever had to play," Pierce said after he and Garnett were treated like rock stars at TD Garden, receiving raucous ovations and emotional video tributes that nearly moved them to tears before the Nets beat the Celtics 85-79 in front of a sellout crowd.
"It was tougher than any championship game. Tougher than any Game 7. This game was just so hard to really just focus and concentrate on what was at hand."
Garnett felt the exact same way.
"This was by far the hardest day I've had to focus," Garnett said. "This was bigger than Minnesota, even when I went back to Minnesota."
Garnett called the Celtics' treatment of him and Pierce Sunday night "over the top."
Pierce played the first 15 seasons of his career in Boston, becoming the second all-time leading scorer in franchise history behind only John Havlicek, while Garnett played six seasons there and made an immediate impact. Pierce, Garnett and Ray Allen teamed up to form the "Big Three" in 2007-08, and ended up capturing the championship that season, their first season together.
Pierce and Garnett were dealt to the Nets in the offseason. Pierce had to convince Garnett not to retire and waive his no-trade clause and join him in Brooklyn. It worked, but the two looked shell-shocked as they sat at the podium and were introduced as the newest members of the Nets.
What began as a struggle quickly turned into a success story, with Pierce and Garnett playing a vital role in Brooklyn's turnaround. The Nets (20-22) started the season 10-21, but have gone 10-1 since the start of 2014.
Pierce had hoped he'd be able to make his emotional return to Boston earlier in the season, but the NBA schedule-makers weren't that kind. In the end, though, it was worth it.
"Paul and I were joking before the game, who was going to tear up and drop a tear," Garnett said. "I had lumps in my throat, but I kept it under control, and tried to focus as much as I could."
Pierce was closer to breaking down, much like a woman who was shown on the Jumbotron holding his No. 34 jersey and bawling her eyes out.
"I did notice that. It was tough. It was tough for me to swallow. I was probably like five seconds from shedding [tears]. I was five seconds, I'll admit to it," Pierce said.
While Garnett said what they accomplished as Celtics will live on forever, he and Pierce knew Sunday night's game was going to be weird. And it was.