BOSTON -- Boston Celtics legend Paul Pierce believes Kevin Garnett's No. 5 will soon hang alongside Pierce's No. 34 in the rafters at TD Garden.

After Pierce's number was retired Sunday in Boston, the 10-time All-Star wondered if, in an era of players switching teams in free agency, it might be a while before Boston's next number retirement. Pierce later told ESPN that he believes Garnett is a slam-dunk to join him.

"Without a doubt, KG's number will be [the next one] retired in Boston," said Pierce. "It's going to happen."

Celtics president of basketball operations Danny Ainge said the team has had informal discussions about retiring Garnett's number, but that a final decision would come from ownership.

Pierce, who played his first 15 seasons in Boston, signed a one-day contract with the Celtics last summer in order to retire as a member of the team. Celtics co-owner Wyc Grousbeck later called Pierce to tell him the team planned to retire his No. 34 this season.

Pierce's was the 22nd number retired by the Celtics. Some argue that Garnett, who played six seasons in Boston after being acquired from the Minnesota Timberwolves, didn't have a long enough tenure to have his jersey retired.

"It was six years of potential championship-caliber teams and All-Star play from KG," said Ainge. "I think that it was a pretty significant impact he had in those six years."

Garnett is credited with changing the culture in Boston. The arrival of Garnett and Ray Allen helped the Celtics vault from a 24-win team at the bottom of the East in 2006-07 to a 66-win squad that brought home Boston's 17th title a year later.

It's telling that the team has yet to put Garnett's No. 5 jersey back into circulation since his departure in 2013. Gordon Hayward took Ray Allen's No. 20 jersey when he signed as a free agent last offseason.

Paul Pierce, right, told ESPN he believes the Boston Celtics will next retire the jersey of former teammate Kevin Garnett. AP Photo/Charles Krupa

There is one opening on an eight-square banner that Pierce's number joined Sunday. Brian Scalabrine, a reserve on the 2008 title team and now a TV analyst for the Celtics, said he thinks it's only fitting that Garnett and Pierce share a banner together in the rafters.

"They go together. Those two guys are linked. That's how it is," said Scalabrine. "Even the fact that there's one more spot on that banner, that spot is No. 5. He's the guy. They hang out [together in Los Angeles], they also hang out right next to each other on the banner. There's no argument out there that he does not belong on that banner. No one out there can make that argument."

Scalabrine also noted how Boston was able to accelerate through its most recent rebuilding process with the collection of picks and assets the Celtics got in return for trading Pierce and Garnett to the Brooklyn Nets. Keeping Pierce and Garnett together on the banner would also reflect their impact on the team even after their departure.

Current Celtics big man Al Horford believes Garnett has earned a spot on the banner.

"He deserves to be up there," said Horford. "Huge impact. In my eyes, from the outside looking in, he made everything happen. He was the type of glue guy, he was a leader, he was the guy that changed everything. It takes a lot of pieces. I know he's never going to want to take the credit, but I feel like his number should be up there. There's no question about it."

The hardest part for the Celtics might simply be getting Garnett to endure an entire ceremony in his honor. But Pierce thinks Garnett would be open to the idea, especially given his affinity for the organization and the championship he won here.

"[Garnett would] rather have his jersey retired in Boston than in Minnesota," said Pierce. "He'll eventually have his number retired with the Timberwolves, too, but he'll have to wait until management sells the team."

ESPN's Jackie MacMullan contributed to this report.