It went from an inevitability to a major whiff.

One moment, Donovan Mitchell was going to be headed home to play for the Knicks. The next, he was en route to Cleveland, leaving Knicks president Leon Rose and company empty-handed.

The Knicks spun the near-trade as a decision to build from within, to save their assets for the next big star. The ensuing months worked out better than anyone could’ve expected.

Jalen Brunson surpassed all reasonable expectations. Julius Randle recaptured his All-Star form. Young players such as Immanuel Quickley and Quentin Grimes took major steps forward. Rose traded for Josh Hart, an egoless two-way player who fit Knicks coach Tom Thibodeau’s system like a glove.

Now, after a successful regular season and an unexpected fifth-place finish in the Eastern Conference, comes another major test: Besting the player who got away. It is the overwhelming focus of one of the most interesting matchups in the first round of the NBA playoffs.

“Life has been full of, especially this year, a lot of full-circle moments for me,” Mitchell said Friday, on the eve of the best-of-seven showdown between the Cavaliers and Knicks that will tip off Saturday night at Rocket Mortgage FieldHouse. “Yeah, we’re playing the Knicks, we’re playing the hometown team, after this whole summer.”

Mitchell is clearly the key figure of this series, followed by Brunson and Randle, whose status is in question due to a sprained left ankle.

The most intriguing matchup will take place when the Knicks have the ball. The Cavaliers have the league’s best defense — first in defensive rating (109.9) and points allowed per game (106.9) — while the Knicks’ efficient offense was tied for third in offensive rating (117.0) and fourth in turnovers committed (13.0). It is the only opening-round series that includes a top-five defense against a top-five offense.

Cleveland, led by dominant rim protectors Jarrett Allen and Evan Mobley, is exceptional at defending the paint. The Cavaliers held opponents to 61.4 percent shooting from 5 feet and in, tied for the third-lowest figure in the league. The Knicks are at their best getting into the lane, with Brunson, Randle, RJ Barrett and Quickley all capable of creating near the rim. They shot 47.5 percent from 5-to-9 feet, the third-best mark in the NBA.

“They’re great [at] defending,” Brunson said. “They have a lot of length and guys that protect the rim. A lot of guys over there that are great defenders. So their defense is incredible. We’re going to be put in positions where we can succeed. We’ve just got to believe in each other. That’s really all it is.”

The Knicks won the regular-season series, 3-1, and were able to make winning plays down the stretch in their three victories. Neither team was at full strength in the last meeting, just two weeks ago. Randle didn’t play for the Knicks, while the Cavaliers were without Allen and top perimeter defender Isaac Okoro (left knee soreness). Randle and Okoro might not play in Game 1; neither has practiced fully this week and both are listed as questionable.

Their absences would obviously be significant. Brunson torched Cleveland for 48 points in the most recent encounter, taking advantage of Okoro being out. Mitchell is obviously well aware of Brunson. He helped end Mitchell’s season with the Jazz last spring, averaging 27.8 points in a first-round series victory while a member of the Mavericks.

“Jalen is the head of the snake,” Mitchell said.

But not having Randle would significantly impact the Knicks. He is their leading scorer and rebounder, and his loss would enable the Cavaliers to focus even more on Brunson. Perhaps because of the uncertainty over whether Randle will play, the Knicks are heavy underdogs. The Cavaliers have the best player in the series, Mitchell, and they have home-court advantage after going an impressive 31-10 in Cleveland. The Knicks, though, were one of the best road teams in the league, with a 24-17 mark, their best since 1997.

“We gotta play tough. We gotta play smart, and we gotta play together,” Thibodeau said. “You can’t do it individually. You have to do it collectively.”

That’s the best way to sum up this Knicks season. It has been a collective effort. They didn’t land Mitchell, and instead relied on development and a few key additions that have panned out better than anyone could’ve expected. It worked out to the tune of a surprising 47-35 campaign, similar to two seasons ago, when the Knicks crashed and burned in an opening-round series loss to the Hawks.

They insist that this team is different. That it is capable of going on a long playoff run. Donovan Mitchell, the player the Knicks had sought for most of the summer, is standing in their way.