It will easily be the biggest storyline in this game despite the heated history between these two squads.
Allen was a member of the Celtics' championship team back in 2008 and was a huge fan favorite in Boston for five seasons. Also, it was only last year that Allen was a part of the losing side of the Eastern Conference finals against the Heat, also as a member of the Celtics.
But after a tumultuous offseason that saw Allen jump ship to the Celtics' hated rival, there has been a ton of negative energy thrown Allen's way.
And Garnett wasn't exactly willing to acknowledge Allen during the pair's first meeting in Miami earlier this season.
It remains to be seen how fans will react to Allen when he comes back since this is his first game in Boston while wearing a Heat jersey. Chances are, it will be positive because despite his changing of allegiance, not even Celtics fans can ignore everything Allen did for the franchise.
For Allen, this will once again be a great chance to prove that the Celtics made a mistake trying to demote him to the bench in favor of Avery Bradley. It was that demotion that ultimately led to Allen being disgruntled and leaving to Miami for less money.
Not to mention a soured relationship with Rajon Rondo.
The Hall of Fame shooting guard already scorched his former teammates in their first meeting, dropping 19 points in 31 minutes. On top of that, Allen is on the best team in the Eastern Conference while the Celtics are struggling just to get to .500.
At the moment, it appears Allen made the right choice.
Will the homecoming be a distraction for Allen? That isn't likely considering his veteran pedigree.
But while Allen would be happy just getting out of Boston with a win, the human element inside of him no doubt craves the draining of a clutch three-pointer down the stretch of this contest, effectively putting his former team away for good.
No matter how he performs, Allen will have to deal with whatever reaction he gets from players and fans, regardless of whether it's negative or positive. But he can't control that now and Allen should know that when his career is all said and done, he'll be more than welcomed back to Boston.