On the verge of a third straight first-round exit, the Celtics enter must-win mode for Game 3.

It’s a stigma no one may have referenced before this week and this series – the worst No. 1 playoff seed of all time.

The Boston Celtics, dominated in two home losses to Chicago to start their first-round playoff series, now have that insult hanging over their heads. And Coach Brad Stevens, faced with his third straight first-round exit – this time with a team considered better than the last two – has once again discovered that playoff pressure increases on everyone.

“I can’t sit here and sulk today,” he said. “Or worry and be down. We have to look ahead, move forward.”

The Celtics play Game 3 Friday night at the United Center, and one bit of encouragement is their 23-18 regular-season road record.

But Stevens, his career postseason record now a frustrating 2-10, admits that playoff intensity is a far different thing from what faces a good team over the course of the regular season. Everything is suddenly magnified, ratcheted up.

“It may be that when players think of being through (the playoffs) in the past, you appreciate the need to play better. We could be better in a lot of everything right now,” Stevens said.

Boston is the first No. 1 seed to go down 0-2 in a seven-game series. When the Phoenix Suns lost their first two against the No. 8 Los Angeles Lakers in 1993, the NBA only played five games in the opening postseason round. The Suns won three in a row to take the series.

What the Celtics need most, perhaps, is a way to restore their spirit after a horrid Game 2 performance in which Rajon Rondo could be heard telling his Bulls teammates that the Celtics had given up down the stretch of the bad loss.

Avery Bradley talked of how teammates could clearly be seen hanging their heads, and allowing dejection to settle in as the Bulls answered each Celtics run with something better.

At one stage, Isaiah Thomas, struggling with inner turmoil over the death of his sister, broke out of his abstraction long enough to shout at Marcus Smart for driving into three Bulls and putting up a wild shot, instead of making the best available play.

Smart just hung his head while waiting for a Bulls player to shoot free throws.

This, according to Stevens, may be the toughest part of his job right now, propping up the spirits of his slumping players.

“Sometimes that becomes a sign that guys are pressing, and feeling that things cannot go our way,” Stevens said. “Later in the game, when we cut it to one and they went on another run, that can be dejecting.

“The biggest thing at this point is to keep the guys upbeat, possession to possession. That’s the way things can get better.”

Several Celtics talked of the team’s underdog mentality heading into this series, even though they were the conference’s regular-season leaders. But Stevens needs players like Thomas and Jae Crowder to continue thinking of themselves as underdogs.

“It’s got to happen, has to happen,” Stevens said. “That’s the charge our team needs.”

But now the Celtics coach, perhaps with a change or two in the lineup, gets to see who on this roster responds to potential elimination from the playoffs.

“I can’t wait for (Friday) night’s game. I wish it was tonight,” he said Thursday. “There’s a taste in your mouth when you go on one of those spurts, something you try to get back. But you have to handle it, move on from it, you’re on the road, in a hole. That’s when that chip we all have has to come out.”