The ad showed photos of Pierce holding his Finals MVP trophy and Garnett lifting high the Larry O'Brien trophy, celebrating their 2008 championship. It read, "Thank you for your heart, for your passion, for your sacrifice, for playing through the pain, for bleeding green, for honoring tradition, for an amazing ride, for restoring Celtic pride, for Banner 17."
The Celtics also shared the message via Twitter.
Pierce spent his first 15 seasons with the Celtics, leaving as the franchise's second-leading all-time scorer. Garnett was dealt to Boston before the 2007-08 season and along with Pierce and Ray Allen led the Celtics to their first title in 22 years.
Pierce, 35, may be entering the twilight of his career, but joked with a reporter from TMZ that he's far from done.
"Who says it's the end of my career? I plan on playing 10 more years, dude," Pierce said.
Pierce, Garnett and Jason Terry were dealt to the Nets in a trade agreed to late last month for three first-round picks (2014, '16 and '18) along with Kris Humphries' expiring contract, Gerald Wallace, Kris Joseph, MarShon Brooks and Keith Bogans (Bogans via a sign-and-trade).
The deal could not be finalized until Friday because of the collective bargaining agreement.