A 3-2 deficit against a proven champion? Sounds familiar.

This Celtics playoff run has been wildly unpredictable, with ebbs and flows, highs and lows and “yeses” and “nos.”

The only pattern that’s held true throughout is that it’s inevitable for everyone to overreact after each game. Anytime the Celtics win, it’s championship or bust. When they lose, people act like their season is over. Those are the rules, and you have to stand tall enough to ride the emotional rollercoaster.

Each time panic has ensued, the Celtics have calmly provided a counterpunch to keep their season alive. With Game 6 set for Thursday night in Boston, there’s still time for the Celtics to put forth yet another a Herculean effort, but the sand in the hourglass is quickly disappearing.

“Now our backs are against the wall,” Al Horford said. “And we have to see what we’re made of. The challenge begins on Thursday.”

The bad news for the Celtics is that they’re up against a Warriors team with a collective killer instinct and a basketball assassin in Stephen Curry. The good news for Boston is that it’s been here before. In fact, just two series ago, the Celtics rallied from a 3-2 deficit to outlast Milwaukee in seven games and advance.

Everyone thought the Celtics were finished after they dropped a heartbreaker in Game 5, but they responded to win two straight and knock out the defending champs. Can they respond from another 3-2 hole? Only time will tell, but it’s not out of the realm of possibility.

In that Milwaukee series, the teams went back and forth in the first four games and were knotted at two apiece – just like the Celtics were against Golden State. A Game 5 win over the Bucks would have put them in an ideal spot, but the Celtics made life unnecessarily difficult once again by faltering in the clutch.

Giannis Antetokounmpo dropped 40 and Milwaukee outscored Boston, 33-21, in the fourth quarter to steal Game 5. That was viewed as the beginning of the end, but it was actually the beginning of something truly magical.

These Celtics are stubborn. For some reason, they play their best basketball when the odds are stacked against them. It’s like when the younger brother takes a lead in the backyard and the older brother realizes he can’t mess around anymore or the younger brother will be in his ear yapping for months.

In Game 6 against the Bucks, Jayson Tatum erupted for 46 on 17-of-32 shooting, Antetokounmpo had 44 and the Celtics stayed alive with a 108-95 win. Then came “The Grant Williams Game,” when he outscored Antetokounmpo, 27-25, and willed the Celtics to a 109-81, series-clinching triumph.

The Finals have followed a largely similar trajectory, with one star player dictating much of the action but the role players often determining the outcome just as much if not more. While Curry’s play is typically a barometer for how the Warriors fare, it’s imperative that the Celtics keep Golden State’s role players in check. If Curry explodes for 40-plus, that doesn’t mean the Warriors will automatically win.

Tatum may not drop 46 again, but he may not have to. The Celtics have the depth and mental fortitude to withstand an onslaught. They need to re-establish themselves as the more physical team, save their legs for the fourth quarter as much as they can and remind themselves that this is their last chance.

“Your faith gotta be at an all-time high,” Jaylen Brown said. “We got to play as a team. All season long, it’s been us vs. everybody.”

The Milwaukee series proved coming from behind is possible. Teams that go down 3-2 are ordinarily finished, but this Celtics team is anything but ordinary.

They’ve relished the underdog role all season. They’ve picked themselves up when everything seems bleak. They’ve defied the odds time and time again. A championship is still within reach, and that starts in Game 6.

“We better be confident,” Tatum said. “We ain’t got to win two in one day. We just gotta win one game on Thursday. We’ve been in this situation before. It’s not over.”