By Christopher L. Gasper, Globe Columnist

The Celtics have a list of issues so long right now they could fill Jack Kerouac's scroll. Rajon Rondo shouldn't be one of them. He should be a problem-solver, but instead he's still behaving like a problem child.

The restive point guard earned himself a head start to the NBA's All-Star break after he lost his cool in the third quarter of the Celtics' 96-81 loss to the Detroit Pistons on Sunday night. After failing to get a foul call, Rondo fired the ball at referee Sean Wright, forcing the NBA to send Ragin' Rajon into the corner for a two-game timeout.

Rondo turns 26 on Wednesday, and instead of celebrating his birthday by playing against best friend Kendrick Perkins and the Oklahoma City Thunder, he's going to spend it watching a shorthanded Celtics team that has lost four straight (and six of seven) try to survive without him. In a shortened 66-game season that is already slipping away, the Celtics can't afford to sacrifice games, or they'll end up first-round sacrificial lambs for Miami or Chicago.

We're past the point where you can simply ascribe Rondo's petulance to youthful indiscretion. He's in his sixth season in the league. He has played in two NBA Finals. He has been an All-Star. As he likes to point out, he's the only one of his generational point guard peers with a ring. If you want to be considered The Man, you can't behave like a temperamental teen.

For all the questions the Celtics have to answer in the second half of the season, the most pressing one facing Danny Ainge is whether Rondo should be regarded as a future building block or stumbling block?

The 10 games the Celtics have between the start of the second half and the March 15 trade deadline could go a long way to making that determination.

The assumption has been that when the Big Three era faded to black, the team would be turned over to Rondo and the Celtics would rebuild around their All-Star point guard. However, Rondo might be too unpredictable in his mood swings and too predictable in his play to be the Celtics' centerpiece.

The same competitive fire that makes Rondo such a joy to watch can turn into a raging inferno that consumes him and leaves his teammates feeling burned and coaches fuming.

You wish Rondo would channel that competitive conflagration into free-throw shooting, a better jump shot or on-the-ball defense instead of self-immolating displays of defiance.

We've reached a point where it's unclear whether the aging Big Three are still helping him steer clear of his on-court shortcomings or putting the emergency brake on his game.

Rondo is as uniquely confounding a player to evaluate as he is to play against. His talents and weaknesses are equally glaring.

There are nights like his triple-double against the Chicago Bulls (32 points, 15 assists, 10 rebounds) where Rondo is a transcendent force. There are other nights where his limitations are as visible as the Citgo sign, and you wonder why every team doesn't just turn his man into a free safety.

Here are the current numbers of two soon-to-be 26-year-old point guards, Rondo and Point Guard X. Rondo is averaging 14.8 points (a career-high), 9.5 assists, 4.9 rebounds and 1.6 steals per game; he is shooting 48.5 percent from the field, 25 percent from 3-point range and 61.5 percent from the free throw line.

Point Guard X is averaging 15.7 points per game, 7.7 assists, 5.2 rebounds and 2 steals; he is shooting 41.6 percent from the field, .38.3 percent from 3-point range with the same number of 3-pointers this season (57) that Rondo has in his career and 87.6 percent from the free throw line.

Point Guard X is Kyle Lowry of the Houston Rockets. You will never hear Kyle Lowry's name mentioned in any discussion of the top five point guards in the league, rightfully so. But around here we always hear Rondo discussed with such reverence.

To suggest Rondo is closer to Lowry than he is to Chris Paul, Deron Williams, Derrick Rose, Russell Westbrook or Tony Parker is hoops heresy punishable by death by Tommy Point. But what if it's true? No one builds a team around Kyle Lowry.

If it were just Rondo's on-court limitations then you could point out that he still has room to grow as a player, but Rondo can't be counted as a franchise cornerstone until he turns the corner on his contumacy.

During the trade talks of the offseason, teams were turned off by and concerned about Rondo's temperament. That appeared to have been a wake-up call.

On the first day of training camp, while addressing his name being bandied about in the Paul trade talks, Rondo was the anti-Beckett. He said all the right things. He struck a more mature, self-aware, self-effacing, measured tone.

It seemed like the days of tossing bottles at video screens and sulking if he wasn't playing well were over. But it appears Rondo has relapsed.

Other franchise players have stumbled in the past, included Pierce, who got ejected from the closing seconds of a playoff game against Indiana in 2005 and then in a juvenile display showed up to the press conference wearing a bandage around his head.

Pierce grew out of his peevishness and into his role as a leader. Perhaps Rondo will too.

Rondo just has to hope that Ainge and coach Doc Rivers remain patient paternal figures, and don't determine like they did with Glen "Big Baby" Davis and Delonte West that Rondo should be somebody else's problem child.