He was utterly inscrutable as he left the sideline, the autopsy of the Celtics’ latest late-game collapse having already begun in the mind that lay behind the expressionless face.

Brad Stevens strode with a calm purpose, his suit unruffled, his blue and white windowpane spread collar shirt still crisp, his tie still knotted in place.

The Celtics coach had just seen his team go scoreless for the last 2:29 on Friday night to complete the squandering of a 16-point lead against the Chicago Bulls and accomplish a third straight loss, 109-102 – the ninth such decision in 13 games.

Stevens was deliberate and encouraging in his postgame chat with the media, fairly pointing out his young club’s proximity to winning some of these close affairs but not sprinkling any serious sugar on what is a depressing set of results.

By this point, Stevens’ tie was undone, but he wasn’t.

Later, as he walked the hallway back to his office, he was asked about his demeanor. The second-year leader has been a cormorant riding the waves through a nor’easter.

But you have to wonder if, in an unguarded moment on a drive home, he doesn’t rip out the Tim McGraw CD and play Metallica at top volume, banging his head against the windshield until the airbags deploy.

Stevens cracked a slight smile.

“I certainly have had my moments,” he said. “Yeah, that’s the best way to put it.”

This isn’t easy – not for a guy who lost just 49 games and had a .772 winning percentage in six years at Butler. He is now 29-66 with the Celtics.

“It’s hard, but I knew it was hard,” Stevens said. “So you come in and you throw everything you’ve got into it, and you don’t change that just because things aren’t going your way.”

That doesn’t mean he is the same impassive man behind the closed dressing room door.

“He has his times,” said Avery Bradley, “but at the same time, we’re all trying to stay positive because we’re all just trying to win games.

“I mean, it’s only the beginning of the year. It’s a long season. Coach is one of the leaders on our team as well, so we all have to stick together and just move on to the next game.”

Said Stevens: “I think I save my moments. What I say to them is what I say to them. Sometimes you have to be a little more emotional than when you go and talk in front of the world.”

As he spoke in front of the pens, microphones and cameras yesterday afternoon, Stevens was just minutes removed from watching Rajon Rondo and Jared Sullinger each clang a pair of free throws in the last 1:04.

He held it together admirably, in part because winning at any time will be exceedingly difficult for this Celtics group, and also because what good would it do to lose it under these lights?

“I felt good coming into the game,” Stevens said. “I felt good at the end of the game. I’m not going to lose too much sleep over the ball not going in the basket. I’ll go back and re-watch the execution and the defensive possessions and those types of things, but I felt pretty good about. Hey, we scored 102 points on Chicago, and that’s with an 11-point quarter.

“So we’re doing a lot of good things. We’ve got to finish. That’s the difference between winning and losing.”

There was much talk about whether the players have begun questioning themselves and whether that is contributing to the late-game immolation. But is it bad enough that Stevens is making his own inquiries?

“I’ve got to tell you, we could win by 30, and I’m questioning me,” he said. “So this has nothing to do with that. I don’t change game to game, as far as my own analysis or being overly critical or any of those types of things.”

For now, he is not showing his cards. He is not averse to criticizing his club publicly after poor performances, but as for what goes on within the Celtics, he knows the dressing room walls are insulated.

No word yet on the Metallica thing.