By Tony Massarotti, Boston.com Columnist

The Celtics were getting blasted by the Miami Heat the last time you saw them, last June, in Game 6 of the Eastern Conference finals. That was when the crowd at TD Garden broke into a familiar chant, spontaneously and sincerely, sending the team back to Miami in what would prove the final days of the new Big Three.

Don't you remember?

Let's go, Cel-tics.

“I want to say to all the fans, thank you guys. I’ve never in my life experienced anything like this — in any sport,” Celtics centerpiece Kevin Garnett said at the time. “I’m just truly blessed to be a Celtic and be a part of the city of Boston. That’s what’s up to all the New Englanders around here. It’s [expletive] crazy.”

And so here we are now, five months later, and the Celtics will be back on the parquet floor on Friday night for their home opener against the Milwaukee Bucks. Garnett is back. Paul Pierce is back. Ray Allen is gone. Rajon Rondo has officially been elevated to core status, displacing Allen at the center of a Celtics operation that is, in some ways, amid an indisputable state of transition.

Out with the new Big Three, in with the Big Three III.

Or maybe just the Big III.

No matter the moniker, the challenges for the Celtics are now crystal clear in the wake of Monday's season-opening loss at Miami, and not solely because the Heat remain the class of the NBA. The Celtics have work of their own. What vice president of basketball operations Danny Ainge amassed over the winter was a collection of talent. Over the next several weeks and months, the Celtics must meld into a team, no small task in the current landscape of professional sports.

The defense, after all, clearly needs work. How and where does Jeff Green fit in? Will Jared Sullinger play a lot or a little? And how, exactly, will coach Doc Rivers manage a roster that includes Courtney Lee, Jason Terry and Leandro Barbosa, the last of whom joined the Celtics late and jumped into the offense as if playing in a neighborhood pickup game?

Roles must be determined, chemistry assessed. How these Celtics fit together is perhaps one of the biggest challenges of Rivers' tenure as coach because, for the first time since Garnett arrived here during the summer of 2007, the Celtics' DNA has been significantly altered.

Five years ago, we all looked at the Celtics team the way some now look at the Los Angeles Lakers, who have flanked Kobe Bryant with Steve Nash and Dwight Howard. Before the Celtics played a game, we wondered about their ability to mesh. We had concerns about the bench. But those Celtics bonded almost instantly, winning their first eight en route a sterling 29-3 record after 32 games. In the third game of the year, on a night when eyes opened all across New England, the Celtics stormed to a 77-38 halftime lead against the completely overwhelmed Denver Nuggets, so impressively and seamlessly working together that we had to almost immediately alter our thinking.

Those were the days of ubuntu.

Culminating in Allen's departure over the summer, the Celtics have since undergone massive changes. In retrospect, most recently, maybe we underestimated the impact. Garnett, Pierce, and Allen made such a fluid transformation five years ago that we may have forgotten they were the exception rather than the rule, even at a time when star players seem to change teams far more regularly.

In their first year together, LeBron James, Dwyane Wade, and Chris Bosh were a rather pedestrian 9-8 after 17 games. Bryant, Nash, and Howard have begun 0-2. The point is that chemistry often takes time in the NBA, even when the involved parties are among the most accomplished players in the league.

In the case of the Celtics, the nucleus of this team has been altered more than we think. Garnett and Pierce simply are not the same players anymore, and Rivers has admitted that he must continue to curb their minutes. That means far more time when neither will be on the floor. That will often leave Rondo to play with a supporting cast he is all but entirely unfamiliar with, including Jeff Green, whose misfortunes have limited him to just 27 career regular season games with the Celtics, two of them starts.

Do we understand how new this team really is? Do we? And come playoff time, as was the case even with the 2007-08 team, the Celtics will have to go through a transition again.

By then, of course, the Celtics will be a far different team than they are now, and not solely because Avery Bradley presumably will be back. (The Celtics were a different team with him on the floor last year.) Rivers will have many of his answers by then, will know who fits with whom, and we all will have a far better understanding of where the Celtics area headed in the short term and the long.

In retrospect, the chants of the TD Garden crowd late in Game 6 last spring were a sendoff for those Celtics, whose future was similarly uncertain.

Against Milwaukee on Friday night at the same TD Garden, the Celtics will reintroduce themselves to their home fans, and the bonding process will start anew.