Jaylen Brown, left, Jayson Tatum and the Celtics trailed their second-round series against the Knicks 2-0, despite leading each of the first two games by 20 points. Charles Krupa/Associated Press

When the Boston Celtics left New York last month, they had just finished a season sweep of the Knicks that made it seem as if a postseason meeting would be a simple step on their climb back to the NBA Finals.

Now it’s looking like the one that could trip them up.

In more trouble than they faced at any time last year, the defending NBA champions take a 2-0 deficit into Game 3 on Saturday at Madison Square Garden, where ticket prices are soaring as fans hope to be in the building to see the Knicks try to close in on their first Eastern Conference finals appearance in 25 years.

“We have to go into Saturday’s game and win the game,” Celtics coach Joe Mazzulla said. “That’s just the way it is. We have to win.”

The Golden State Warriors and Minnesota Timberwolves are tied 1-1 in the other series that resumes Saturday in San Francisco, and they have the prime-time spot on ABC’s schedule.

But with Stephen Curry on the sidelines and the Celtics on the ropes, the afternoon appetizer has become the main event.

Boston lost only three games all last postseason. Lose a third straight in this series, and the Celtics would need the biggest comeback in league history to continue their title defense. No NBA team has overcome a 3-0 deficit to win a series.

“The mentality is 0-0. Don’t even focus on that,” Knicks All-Star Jalen Brunson said. “Just focus on the next play, next quarter and don’t look ahead. Don’t look at anything, just try to focus on the task at hand and be present.”

The Celtics should probably be no worse than tied. They had 20-point leads in the second half of both games in Boston, but both ended with Mikal Bridges making a steal to preserve a Knicks victory.

There hasn’t been the same drama in the West series. Minnesota raced to a 13-0 lead and never trailed in its 117-93 victory Thursday, after Golden State led comfortably most of the way in Game 1 even after Curry was lost to a hamstring injury.

“I think both teams are really committed to defense,” Timberwolves coach Chris Finch said. “We for sure have to take all of the easy baskets out of the game. I think we’ll try to figure out a few other things that they’re doing, as you do in a series as it goes along. Of course, defense is going to be huge.”

The Celtics’ problem has been their offense. They are 25 for 100 from 3-point range in the series and have averaged just 16.5 points in the fourth quarter. All-Star Jayson Tatum has missed 30 of his 42 shot attempts.

“I take full ownership on the way that I’ve played in this series and can’t sugarcoat anything,” Tatum said. “I need to be better and I expect to be a lot better.”