Whenever one player’s highlights from a single game take nearly 15 minutes, it’s safe to say that player performed at a high level. An absurd triple-double of 18 points, 20 assists and 17 rebounds was the latest evidence that Rajon Rondo — when operating smoothly — is capable of controlling every facet of a basketball game.

Nothing newsworthy there, though Rondo (and everybody else, really) rarely reaches the all-encompassing greatness No. 9 exhibited during Sunday’s win against the New York Knicks. Rondo scored 18 points and could have had more if he didn’t get a case of the layup yips late in the contest. He grabbed 17 rebounds, or one fewer than New York’s entire bench plus Carmelo Anthony, which played a combined 151 minutes. He dished 20 assists, especially remarkable since his Celtics teammates scored just 38 baskets.

But like with every Rondo masterpiece, the resulting euphoria is clouded by the nagging question that has followed Rondo like a shadow for his entire career. If he is capable of such Oscar Robertsonian accomplishments, why does he sometimes seem to disappear throughout the course of entire games?

Nobody could ever expect Rondo to have 18, 20 and 17 each night, but it’s reasonable to suggest that he should never suffer full games where he scores just as often as the box of Cheez-Its from which I’m currently eating (note: perhaps your Cheez-It boxes are high-scoring, but mine have endured 24 straight years of DNP-CDs). It’s easy to wave your “Never trade Rondo!” flags after his third consecutive double-double was actually one of the most unique triple-doubles ever recorded, but before those three straight outings, Rondo shot 0 fer 6 against Cleveland, scoring zero points while getting simultaneously torched by Kyrie Irving. The Celtics won, which was nice, and Rondo contributed 11 assists, also nice, but he clearly spent the night as the second-best point guard on the floor.

Less than a week later it was Sunday and the Knicks were in town, and Rondo morphed into the planet’s best hooper. Apologies to Mr. 57 Deron Williams, who did his best to steal the day’s title… and probably did, but I’m trying to make a point here, damn it: There are times when Rondo plays like the best basketball player alive. He’s had playoff runs where he nearly posts a triple-double every night for two weeks straight. He defends, he rebounds, he distributes, he scores — sometimes, Rondo does everything but control the scoreboard, keep the stats and sweep the floor. Which is incredibly, spectacularly breath-taking — and if you’re a Celtics fan, awesome — until you remember that every Rondo performance comes with that same asterisk: tomorrow, Rondo might be normal again.

The unpleasant Rondo dichotomy might be due to motivation. Thirteen of Rondo’s seventeen triple-doubles have come during nationally televised games, suggesting that he uses the bright lights as rocket fuel. He also seems to elevate his play in the playoffs, perhaps giving credence to the motivation theory. And Kevin Garnett noted yesterday that he can tell beforehand when Rondo will put forth a big effort; apparently, Rondo gets a different look in his eyes before playing a Sunday tilt with the Knicks on ABC than he does when playing the Cleveland Cavaliers on a Tuesday night.

“I’ll save this just as a witness that I was here and I actually got to see this up front and center,” Garnett said of the box score. “The thing about Lin is, I think everybody who’s at the point guard position, [they are] already excited to play the kid, [Rondo] was nothing short of that today. I can see it and I can tell. I’ve been around him long enough to know when he’s motivated and when he’s more than motivated. Tonight was one of those nights.”

Later Garnett continued, “You could see it on his face. Just like when he was playing Deron Williams [on Friday]. If you’re going to be anything in this league, you’ve got to play against your position. [And] trade talks really are a good motivating factor for him. If you know Rondo, he’s an I’ll-show-you type of person, very motivated, and that’s what you love about him. This game had enough juice on it for him to come out and play the way he played.”

As we’ve witnessed for years, there are times when Rondo dons his cape and becomes Superman. Unfortunately, he sometimes forgets to enter the phone booth and plays as Clark Kent instead.

Personally, I like when he’s Superman.

But if he ever develops to the place where he’s just Rajon Rondo, a steady, do-it-all point guard who doesn’t excel as a shooter but makes up for it everywhere else, on every single night, I would be mighty content.