BOSTON — As Coach Ime Udoka pointed out postgame, the Celtics’ mantra this season has been all about how they have responded to adversity. When the first-year coach called out his team’s lack of mental toughness, the season seemed lost … until it wasn’t.

And every single time the Celtics stared in the Bucks’ face in a crucial game, they delivered. It happened in Game 2, a home win where they blew out Milwaukee. It happened in Game 4, where the Celtics put together a comeback to avoid a 3-1 hole.

Of course, those efforts came crumbling down in Game 5 in a 110-107 loss to the Bucks on Wednesday at TD Garden. The Celtics collapsed in epic fashion, blew a 14-point fourth-quarter lead, botched the offensive rebound off a missed free throw and failed to execute their final few possessions.

They now stare a different foe in the face: Elimination. The Celtics had fans dreaming of something special, and while those images aren’t gone, they are on life support.

“What we’ve been done throughout the series and throughout the season is bounce back when we’ve been tested — and even in this series,” Udoka said. “It was an opportunity (in Game 5) but gotta go to Milwaukee, take it one game at a time and try to bring it back here.”

The Celtics are one loss — or a few bad bounces late — away from a postseason exit. They’re down 3-2 with the series shifting back to Fiserv Forum for Game 6, which is set for 7:30 p.m. Friday. The defending champs stole Boston’s heart when Bobby Portis banked in a putback for the go-ahead bucket.

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The Celtics’ entire season has been about how they bounce back from adversity. They had proven they’re a different team during a dominant stretch in the second half of the season. There were times when the Celtics looked like contenders — even in this series, when they brought home-court advantage back to Boston after an important Game 4 win.

Game 6 will be the Celtics’ biggest test of the season, and in the present, there’s nothing beyond that. There’s no looking ahead, Jayson Tatum said, as they try not to apply any unnecessary pressure to themselves.

“There’s no sense in being sad or putting your head down because that’s not going to do anything for next game,” Tatum said. “Always be optimistic and believe in yourself, believe in your group that we can win the game on Friday.”

The Celtics, of course, have the juice to knock off the defending champs. Games 3, 4 and 5 could have all gone to either team; the Bucks went 2-1 in those contests. Udoka said they should look at the positives, whether it was the solid defense or how they built the lead.

Fixes are necessary, though. The Bucks have dominated the Celtics on the offensive glass, and it was fitting that Portis’ putback proved to be the difference.

“If we boxed out, we win that game,” Marcus Smart said. “That’s what it comes down to. They did a good job of getting extra shots and they hit them. They made us pay. Have to do a better job in that end, clean it up.”

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The Celtics clearly have the mental fortitude to play the Bucks tough on the road. They came back from double-digit deficits in both Games 3 and 4. Just like Smart said, the Bucks “did to us what we did to them the last game.”

A second-round loss to the defending champs is far from a failure — especially considering the 18-21 start to the season. But winning the NBA finals requires some luck and determination. The Bucks are the perfect example from their NBA championship run last season, when they came back from down 3-2 to the Nets then 2-0 against the Suns.

The Celtics still have the mettle of contenders, that reality didn’t change from a stunning loss Wednesday. But if they want to accomplish anything beyond the Eastern Conference semifinals, they need to navigate the improbable.

And the Celtics have proven they can do just that to flip the narrative — now it’s about putting it all together.

“It definitely hurts, but we don’t have time to feel it,” Smart said. “Pretty simple. If you’re not ready for the next game, then don’t step on the court. That’s how you get ready and move on.”