A game that should have been so much about Ray Allen’s return to Boston and the clash of two Eastern Conference rivals is so much not about that.

The reality is, the Celtics are ill-prepared for Sunday’s matchup with the Miami Heat, having lost six consecutive games. At this point, they can’t beat the Pistons or the Cavaliers or the Knicks. Even more ghastly, they couldn’t hold a 27-point lead against the less-than-rugged Atlanta Hawks.

Allen is not vengeful, but his eyebrows are obviously raised at the Celtics’ precipitous decline since his departure. And at how his replacement at shooting guard, Courtney Lee, is now coming off the bench. And at how his replacement as a 3-point shooter, Jason Terry, says he still is uncomfortable with the offense and his role.

Advertisement - Continue Reading Below

Allen returning to TD Garden was supposed to be the story line — if the Celtics had held up their end of the bargain and flourished with their retooled roster. Instead they are a mess, with a befuddled Doc Rivers unsure in what direction to coach.

One night, the bench disappears, and the next, the starters fail to make an appearance. Against the Hawks, the bench handed a 19-point bulge to the starters, who took merely six minutes to blow it.

After threatening to trade everyone except trainer Ed Lacerte and PR guy Jeff Twiss last Sunday, then watching 20-year-old Cavs rookie Kyrie Irving cement his All-Star status with a 40-point game Tuesday, then looking on as Paul Pierce and Terry committed deflating turnovers against the Knicks, Rivers was left trying to explain the loss in Atlanta.

The issue of Allen coming back to Boston couldn’t be more of an afterthought. Rivers looks exhausted. He looks weathered, and we’re only in January.

Celtics fans always have known Doc Rivers to be like Dr. Phil, someone who always had the answers. Even after the toughest losses, he could come up with a rationale that would salve the wound.

But on Friday, he was pretty much like the rest of Celtics Nation.

“Some of the tough stretches since we’ve had this group have been self-inflicted,” he said. “This stretch isn’t that. This stretch is we’re ready to play every night and not ready to play every night.

“Again, I’ve got to do different things. I don’t know what they are, clearly. But I’ve just got to keep searching with the team. You’ve seen the flashes, but we just can’t sustain. I don’t know what it is. But we’ve got to figure it out.”

Even when Allen was here, the Celtics had a bad habit of playing down to their opponents, but they always managed to rally back.

According to Rivers, his coaching colleagues tell him, “Oh, you know how your guys are — you guys don’t ramp things up until after the All-Star break anyway.”

But Rivers, generally the problem solver, always the voice of reason, sounds more distressed because only four players remain from those teams.

“We have nine new guys here,” said Rivers. “They’ve never done this. They may be thinking [we can just rally], but if they are, it’s an error.”

Although the Celtics have been getting old, there was much to be said about their experience and ability to gather themselves. This bunch appears shell-shocked, unable to fathom why winning is so difficult, why opposing teams — even the lower-tier ones — don’t succumb at the sight of the green jersey.

The Heat stopped fearing that color years ago, and LeBron James and Dwyane Wade would like nothing more than to get Allen a win in his return by stomping the Celtics on national television, adding further embarrassment and tarnish to their image.

The answer to the Celtics’ issues is inside their locker room. They can’t look to Rivers because he no longer has the solutions. They can’t look to Allen, because his leadership and resolve left along with his 3-point shot.

The Celtics are faced with the ultimate challenge. How do they overcome themselves — their insecurities and shortcomings — for a second-half run? What’s done is done; Allen is gone, angry that the Celtics called Terry at midnight at the start of free agency when they already had a 3-point shooter at their disposal.

The Celtics can’t concentrate on Allen or even the Heat, they have to concentrate on righting themselves, restoring their pride and passion to win.

How? Nobody has that answer so far.

“This [expletive] is frustrating, though, I’ll be honest,” said Kevin Garnett. “This is the season, man. It’s going to have a lot of ups and downs, peaks and valleys. It’s time to stay together. More important, it’s time to work.

“We can’t come out here and keep making the same ol’ mistakes, man. It gets old.”