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.
"We had dinner with [former teammate Rajon] Rondo last night," Pierce said. "It was good to see him. I couldn't think about anything but today, really. It was hard for me to sleep, just laying in a downtown hotel in Boston, when I'm used to being at my house. Getting into the arena, coming through the back side, making a left [to the visitor's locker room] instead of a right. Just everything was so different, and it was great, though."
Pierce and Garnett were showered with ovations from pregame warmups on, and those ovations reached a crescendo when each of their moving tribute videos was shown on the Jumbotron -- Garnett's late in the first quarter and Pierce's at the end of the opening quarter.
Each time, the videos, which lasted about a minute long, showed the banners of retired uniforms hanging in the rafters above the parquet floor. It's only a matter of time before the empty spots belong to Pierce and Garnett.
"The one word that comes to mind is 'unbelievable,' " Garnett said. "I didn't expect anything like that for myself. It shows the first class, it shows what type of organization this is and their appreciation for you. And I couldn't put it into words."
They might play in Brooklyn now, but Pierce and Garnett both admitted they'll be Celtics for life -- and appreciate everything the fan base has given them over the years.
"I just wanna give a special thank you [to the fans]," Pierce said. "Through my bad times, through my immature times, through my growing up and becoming a man in this city, to winning a championship and everybody sticking with me and standing behind me, I'd just like to tell them thank you."
Added Garnett: "I think we will always bleed green, as long as we're playing basketball. As long as we're living, even when they bury us six feet [under], it's what it's gonna be."
When Pierce and Garnett were introduced in the starting lineup on the public address system, the fans roared. Garnett acknowledged the crowd, while Pierce elected not to, urging his teammates to begin their pregame dancing routine. The emotion clearly was getting to him, and he didn't want that to happen.
Pierce and Garnett had little impact on the game for the most part. Garnett, though, did make one of the biggest plays of the night. With the Nets nursing an 80-77 lead, he came through with a steal at half court and went in for a layup with 17 seconds left, making it a five-point game.
"Well I knew they were trying to run a pick and roll and Rondo was trying to get a 3 at that point," Garnett said. "I just played the passing lane. It took me two days to get the layup up. I thought I was gonna get caught there for a minute, but I got it still, put the ball in front of me and got a layup."
Pierce finished with six points on 2-for-10 shooting in 29 minutes, while Garnett had six points, three rebounds, three assists and three steals in 23 minutes.
Garnett said the magnitude of the night "didn't really hit him" until he saw members of the Celtics organization -- people like trainer Ed Lacerte, VP of public relations Jeff Twiss and director of team security Phil Lynch, among others.
"The guys who people don't really know but make this thing really go around," Garnett said.
"I saw so many friends, so many people I've known throughout the years," Pierce said. "Then you get showered with love throughout the game, you see so many Garnett jerseys, my jerseys and it's every second you're on the bench, every second during the game, people are calling your name. It was tough. But I'm happy we got it over with and I can get back to playing basketball now."
And he's doing it with someone who he feels is "like a big brother to me" -- Garnett.
"If our ride is gonna end here in Brooklyn, I wouldn't want it to end with anybody else," Pierce said.