Only a few days after the Jets fired him after his seventh season as the G.M., on Jan. 2, 2013, Tannenbaum received a call from Bartelstein, an influential agent based in Chicago. They had worked together on deals, and their relationship was good. Bartelstein wanted to start a branch of his company to focus on nonathlete clients, and he recognized in Tannenbaum — a numbers cruncher with accounting and law degrees — a unique opportunity.

“He has a view of the business unlike anyone else because of his past experiences,” Bartelstein said, adding, “I’m also a huge believer in anything you do in business, it’s tremendously beneficial if you have a view from the other side of the table.”

Tannenbaum, who began doing on-air work for NFL Network, said he took a few months before deciding to join Bartelstein. He fielded inquiries from other teams about front-office roles but liked the idea of something different.

“I felt like I want to own something, and I want to do it with good people, and I didn’t want to relocate,” Tannenbaum said. “Those were three really important factors for me.”

He began working out of his house in Basking Ridge, N.J., where his first employee, a former Jets intern named Aaron Steeg, sat in the dining room and sneaked food to Tannenbaum’s Labrador, Larry, under the table. After a few months, office space was necessary, and Tannenbaum added a third employee, Marc Barone, a former Wall Street trader with no sports background.

Bartelstein introduced Tannenbaum to Kerr, one of his longtime clients, at the Final Four in 2013, and from there, Tannenbaum began gearing Kerr up for the 2014 job-hunting season. He and his staff gathered data on every coach’s contract in the N.B.A. and put together a 41-page manifesto detailing how Kerr would run a program.

When Mike Woodson was fired by the Knicks, Kerr’s relationship with Phil Jackson, the new president, made that franchise an early front-runner. Then Mark Jackson was fired at Golden State, and Kerr also had a relationship with the Warriors’ owner, Joe Lacob, and the president, Rick Welts, who worked with Kerr with the Phoenix Suns. Suddenly, Tannenbaum had a bidding war at his fingertips.