Minus Rondo, these Celtics are done

The Boston Celtics swiped a double-overtime win over the Miami Heat without Rajon Rondo on Sunday, but the knee injury that led to Rondo’s absence — and the fact that it’ll keep him sidelined for the remainder of the season — could be a precursor for disaster, a harbinger for change or both for a team already in danger of missing the postseason for the first time since 2007, before the Celtics started the Big Three movement.

Rondo, the NBA’s assist leader and a fan-voted starter in next month’s All-Star game, initially injured the knee Friday in a double-OT loss to Atlanta. In that game, the sixth straight defeat for Boston, the Celtics blew a 27-point first-half lead despite getting a second consecutive triple-double from Rondo, who leads all players with 23 such games since the start of the 2008-09 season, including 10 in the playoffs.

After feeling pain in the knee during Sunday morning’s shootaround at TD Garden, Rondo went to the hospital, where tests revealed that he had torn his right ACL.

The prognosis isn’t good, of course, nor is it ever when a player sustains an injury that severe. Even the absolute best-case scenario would put Rondo back on the floor sometime during the first half of next season — though the precedent with this type of injury suggests it could be longer. That gives the Celtics about a year to figure out their next move, because they can’t stand idly by with Rondo on the shelf.

If Boston has any aspirations of competing this season or at any point going forward, it needs to have a serious discussion about the direction of the franchise, and it might be in the team’s best interests to start looking at alternatives for what they already have. Because the inevitable demise of the Celtics team that has been the East’s most consistent over the last half-decade could finally be a reality with Rondo out long-term.

Essentially, the Celtics have three choices as they analyze their immediate future, and their long-term status:

1) They can maintain the status quo and continue on the track that had them barely holding onto the eighth seed in the East even with Rondo, putting the rebuilding discussion on hold for another year.

2) They can try to trade for a game-changing playmaker to fill in for Rondo and hope that it’s enough to stay competitive in a race for second place that has no clear frontrunner.

3) They can blow up the entire operation and reconstruct, a thought they’ve been flirting with for what feels like forever.

None of them is particularly popular, but only one — doing nothing — is a virtual certainty to fail.

A quick look at Boston’s win-loss record without Rondo over the years would have you believe things are OK with him out, as the Celtics have actually gotten by fine without their sparkplug guard in the past. After Sunday’s win, Boston is 25-16 overall with Rondo out since he became the team’s full-time starting point guard in 2007.

But this year’s team is a year older than the team that was already too old last year, and they’ve done little to suggest that somehow they’d be better without their best player on the floor from here on out. The idea of rallying around a fallen comrade works in the movies, but I don’t see this year’s group having any more of the kind of fairytale magic that carried them to within one win of last year’s NBA Finals.

If they choose to take that route, though, the Celtics would almost certainly have to revert to the strategy they used last January, when Rondo missed eight games with an injured right wrist. During that span, which saw the Celtics go 6-2 after a 5-8 start to the season, Avery Bradley stepped in as the team’s starting point guard — serving more of a defensive role than anything else — as ball-handling duties were turned over to Paul Pierce.

In that stretch, Pierce averaged 21.5 points on 47.9 percent shooting and dished out 7.5 assists to go with six boards a night, all up from his season averages. Pierce put some of that versatility on display Sunday, with 17 points, 13 rebounds and 10 assists in the win. But with Boston shorthanded for the foreseeable future, the team’s longest-tenured player is going to need to be as crafty as ever, and I’m not sure he’s prepared to carry that load.

The Celtics do have one thing going for them, and that’s the conference they play in. As of Sunday, Boston is 21-23 — three games back of Milwaukee for the No. 7 seed in the East, and the only thing saving them from missing the playoffs could be the unthreatening gaggle of teams on their heels. The Celtics are currently 2-1/2 games ahead of Philadelphia, four games ahead of Detroit, five games ahead of Toronto and 6-1/2 games ahead of the Magic.

So a playoff spot seems reasonable for the Celtics. However, a series win is anything but, and that’s why they may need to make a trade if they want to be competitive once they get there.

That’s easier said than done, though. The market isn’t exactly flush with elite backcourt options, and certainly there’s no one available and easily attainable who can match Rondo’s skillset and ability to quarterback the game. So some very careful thought should be put into who, if anyone, would actually make the desired difference right now without complicating the long-term plan — which is where the team’s spotlight should ultimately be shining.

Trade or no trade, the Celtics can’t compete with the Heat or any of the Western Conference’s powers this year, so they might as well put the time and effort into figuring out how to get there in the years to come, because the other option is fading into the ether. Last year, Celtics GM Danny Ainge toyed with the thought of rebuilding, and this is the perfect opportunity to actually do it, frustrating as it may be for fans.

Pierce and Kevin Garnett are too old to build a team around, but they have enough value that dealing them would be worthwhile, and role players like Jeff Green, Brandon Bass, Jason Terry and Courtney Lee could fit well on a contender with a particular need and draft picks to shed. As it stands right now, the Celtics have enough assets to give themselves a chance to retool around Rondo when he returns, and it’s an opportunity they won’t have for long, so they should act.

The latest incarnation of the fading Celtics was only a marginal contender with Rondo to begin with, and it won’t be one at all without him. And while no one would ever wish for Rondo to go down like he did, Boston needs to make the most of its situation and make a long overdue move with the future in mind. The result won’t be pretty over the next few months or even the next couple years, but it’s the right strategy for a team that’s dangerously close to becoming irrelevant.

You can follow Sam Gardner on Twitter or e-mail him at samgardnerfox@gmail.com.