TORONTO — During the Knicks’ rushed December training camp, interim general manager Glen Grunwald brought coach Mike D’Antoni into his office at their Greenburgh practice facility for daily meetings.

On the wall in Grunwald’s office is a greaseboard. During the frenzied free-agent period, the names of players put on waivers were listed on the board. Grunwald and D’Antoni discussed the players’ merits.

On Dec. 24, Jeremy Lin hit Grunwald’s greaseboard after he was cut by the Rockets. D’Antoni and Grunwald discussed Lin’s impressive pre-draft workout in Greenburgh on June 7, 2010, his penetration ability and knack for the pick and roll.

Grunwald had recommended bringing Lin in for training camp last season and earlier in December, but Golden State and Houston had gotten in the way.

“I remember Glen saying after Lin got waived, ‘He can run the pick and roll and be a playmaker better than anybody we had,’ ’’ D’Antoni recalled.

But the Knicks had enough point guards on the roster by then. In the season opener on Christmas Day, rookie Iman Shumpert sprained his knee and went out for at least two weeks.

The decision was made the next day when Grunwald told D’Antoni this was the Knicks’ chance to claim Lin — at least as a stopgap because his $788,000 contract wasn’t guaranteed. After 16 teams passed in waivers, Grunwald sprang.

“Lin and [Steve] Novak, they were both Glen’s calls,’’ said a person familiar with Grunwald’s daily waiver talks with D’Antoni.

Earlier that month, D’Antoni was more impressed by his new boss during the hectic period when the franchise did mathematical cartwheels — a series of maneuvers that began with Chauncey Billups’ amnesty waiver — to open enough salary cap space to sign center Tyson Chandler.

“Trying to get Tyson in those few days, his demeanor to deliver under enormous pressure was terrific,’’ D’Antoni said. “The way he went about it: calm, cool, collected through the whole thing.’’

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Lin has morphed into a global sensation, suddenly a recognizable face around the world as the Knicks prepare to face the Raptors tonight in Toronto.

Grunwald? Most Knicks fans couldn’t pick him out of a lineup. He has remained invisible — despite his 6-foot-7 height — working behind the scenes in advancing former team president Donnie Walsh’s rebuilding agenda.

However, Grunwald changed Walsh’s master plan on a dime, thinking out of the box to heist Chandler from the Mavericks, being bold to claim Lin and sticking behind D’Antoni’s system during the Knicks’ slump. Even Novak, who was put on waivers Dec. 19, is looking like a find, with Grunwald seeing him as the perfect 3-point shooting ace for D’Antoni.

Grunwald’s moves have for the time being made the Knicks the talk of the sporting world — if not an NBA title contender.

“How would you ever predict that?’’ Indiana coaching legend Bob Knight replied when asked if he saw a future NBA executive in Grunwald when he coached him at Indiana. “He was a very, very bright kid. Anything he went into, anything he wanted to do, I felt he could do.’’

Since the preseason began, Grunwald, 53, has not spoken to one media outlet. The Knicks declined to make him available for this story.

“Some general managers like to be seen and heard,’’ said Knicks assistant coach Mike Woodson, who played with Grunwald at Indiana. “Then there are some GMs who like to stay in the background and make sure things get done the right way.’’

Grunwald’s predecessor, Walsh, spoke to the media daily, perhaps too much for owner James Dolan’s liking.

Grunwald seems to prefer it this way, though it was not his modus operandi in Toronto, where he was always available to the large media contingent and remains much more famous than he is in New York.

When Grunwald finished his seven-season stint as Raptors president, he became CEO for the Toronto Board of Trade and wrote a daily blog.

Former Raptors owner John Bitove is not surprised Grunwald has laid low, even if it’s not his call. Bitove believes Grunwald is abiding by Dolan’s preference for executives to have their actions speak louder than their words.

“He doesn’t care for the spotlight,’’ Bitove told The Post. “What matters to Glen is doing the job exceptionally well. He does it how Dolan likes to do things. Glen’s a perfect fit. He doesn’t care or need his name in the paper.

“There’s no one more loyal or smarter than Glen,’’ Bitove added. “If I was doing something in business, I’d bring in Glen. He’s not just basketball-smart. He’s smart in anything.’’

The Chandler transaction — in which Grunwald blew up the Knicks’ 2012 cap space due to their diminishing chances of landing point guard Chris Paul — was not the first time Grunwald thought out of the box.

In fact, the Raptors should call their practice digs at Air Canada Centre “The Gym That that Grunwald Built’’ — and the Knicks financed.

After Bitove left as owner, the Maple Leafs ownership group purchased the basketball team.

“For a while it was all about hockey, and basketball was a distant second,’’ Bitove said.

When the Raptors were building a new arena in the late 1990s, Grunwald campaigned for a practice facility to be built inside. The cost was projected to be $2 million, and new ownership rejected it.

Grunwald found a way. In trade negotiations for Charles Oakley in 1998, he got the Knicks to throw in $2 million in cash.

“Glen told ownership, ‘I’m doing this trade on one condition: We use the money for the practice facility,’ ’’ Bitove recalled. “That’s a classic example of getting it done and never tooting his horn. He just wanted what was best for the team.’’

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Grunwald’s basketball career at Indiana was a big disappointment. Recruited by Knight as a four-time high school All-American and the No. 1 recruit from Illinois, the Chicago native tore his ACL playing pickup before his freshman year with the Hoosiers. Grunwald still played four seasons, but was a fringe player, appearing in 103 games (28 starts) and averaging 2.4 points.

Knight blames Grunwald’s lack of playing time on “a respiratory problem’’ that hurt his stamina. Everyone else remembers Grunwald losing all mobility because of the knee injury.

“His knee wasn’t right,” Woodson said. “He pretty much hobbled the years we were together. I remember he wore a brace. It was unfortunate he didn’t get a chance to display his abilities.’’

Said Knicks assistant coach Herb Williams, who faced Grunwald playing for Ohio State, “I think he would have been a pro if he stayed healthy.’’

It wasn’t a total loss. Grunwald won a national championship in 1981, and forged a friendship with Indiana star Isiah Thomas, who remembered his former teammate’s smarts and hired him in Toronto in 1994 when he became Raptors president.

Grunwald started as VP of legal affairs. After Indiana, Grunwald had gotten a law degree at Northwestern. His wife, Heather, whom he met in Toronto, is a high-powered lawyer in New York.

When Thomas moved to the Knicks bench in 2006, he hired Grunwald again to be his assistant GM and right-hand man. The Grunwald-Thomas relationship is a sensitive topic because of a perception Thomas, currently the coach at Florida International, is constantly in Grunwald’s ear. However, their business relationship is not close, though Woodson said the trio got together with five other Knicks staffers for a dinner on Jan. 26, the night before a Knicks-Heat game in Miami.

“You’re talking about 30 years of friendships there,’’ said Woodson, who said he came to the Knicks because of Grunwald. “Isiah had an off day from his team. We had to go get dinner. It was nice. We laughed and talked a little basketball and moved on.’’

Bitove, who hired Thomas, said, “I think Glen’s smart enough to have a bunch of people he calls from time to time to bounce things off. Sometimes he takes advice. Sometimes he doesn’t. I don’t think Glen needs a big brother anymore.’’

Bitove was not surprised the well-educated Grunwald joined Toronto’s Board of Trade after he was fired by the Raptors. But Grant Humes, COO of the organization, expected a different person.

“When I first met him I was shocked,’’ Humes said. “The impression of sports figures is they are boisterous. He was thoughtful and quiet, to the point of being shy. He was in the Toronto media all the time, but really was a quiet man.’’

Considering the Raptors’ deep spiral since Grunwald’s exit, his years are remembered fondly in Canada. The Raptors made the playoffs three straight seasons under Grunwald. That his 18-year-old son, Gabe, is a star goalie for a junior-league hockey team in Michigan gives him cachet north of the border.

“They were the best they ever were, and he loved this city,’’ Humes said.

“Interim’’ still is attached to Grunwald’s general manager title, and nobody has a crystal ball on where his Knicks are headed. But as the world recognizes Lin and the Knicks, they would be good to remember Grunwald, the quiet man who picked him out off the greaseboard.

marc.berman@nypost.com