Though everything would suggest otherwise, Ray Allen insisted last week that when he returns to Boston’s TD Garden on Sunday for the first time since joining the Heat, the Celtics will be “just another opponent.”

But they cannot be.

Not with the history here.

Not with what he accomplished there: an NBA title and three All-Star appearances in five seasons in Celtics green.

Not with the prickly feelings between Allen and Rajon Rondo.

And not with all the noise surrounding his departure, which included coach Doc Rivers claiming the reason Allen left “was ego more than anything else. … Ray wanted the ball more.”

Allen said he doesn’t know precisely how he will feel when he steps on the court hours before the Heat and Celtics meet at 1 p.m. on ABC.

“I will deal with whatever emotions there are,” he said. “You never know until you get there. Regardless of how they respond to me, it won’t change how I feel. I have great memories.”

LeBron James, Chris Bosh and other Heat players said Allen should expect to be booed.

“I don’t expect it to be good,” said James, booed heavily when he returned to Cleveland as a member of the Heat. “If you don’t wear green in that building, they don’t like you. We’re there for him.”

Heat coach Erik Spoelstra cracked: “Maybe the highest form of compliment would be for them to boo and throw stuff at Ray.”

Celtics coach Doc Rivers hopes that isn’t the case. “I hope he gets a good round of applause,” Rivers told Fox Sports Florida. “He did a lot for this franchise. I know he’s a great guy.”

Bosh, booed when he returned to Toronto for the first time after joining the Heat, said Allen should expect a “very emotional” day.

“All these emotions are pent up and brewing,” Bosh said. “But it helped me focus in a lot more. The key is just play the game.”

Said Dwyane Wade: “We just want to play well for him.”

Allen insisted last week he has nothing but “positive” feelings for the Celtics, for whom he made 798 three-pointers and scored 5,987 of his 23,384 career points.

“Everything I accomplished there and experienced there brought me to this place I am now,” he said. “And I’m appreciative for all of it, because the negative you learn from just the same.”

The Allen/Celtics relationship was far from warm and fuzzy in his final months as a member of the team or in the aftermath of his exit. It has been well documented that Allen was upset not only about being replaced in the starting lineup, but also that the Celtics nearly traded him to Memphis.

That, coupled with his frosty relationship with Rondo, made it easy for Allen to reject a two-year, $12 million offer from Boston and take a two-year, $6 million deal with Miami, including an opt-out after this season.

Though Allen has suggested that stories about friction with Rondo are overstated, Sports Illustrated reported that Allen and Rondo had to be separated in the locker room at least once because Allen believed Rondo was intentionally looking him off.

Rondo hasn’t referred to Allen by name in months, calling him merely “that guy.” Kevin Garnett said he intentionally “lost” Allen’s phone number.

And in the season opener in Miami, Garnett ignored Allen when Allen wandered over to the Celtics bench to quickly greet him and slap him on the shoulder.