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In making his most important decision as the Nationals’ general manager, Mike Rizzo relied on the force that, for better or worse, drives the franchise: faith in himself. Rizzo did not choose Matt Williams to manage because he knows Williams will be great, that he will be the kind of leader who can guide a talented roster back to the playoffs and into the World Series.

The Nationals will hire Williams not because of hard evidence he can do the job — there isn’t any — but because Rizzo believes Williams will be a good manager. He came to know and admire back in his days as Arizona’s scouting director, and he kept him in mind all these years. His belief in the man — which means, really, his belief in his ability to evaluate Williams — in his mind must have obscured any risks.

And there are risks. Williams will take over a ready-to-win club with no intimate knowledge of the personnel except perhaps Anthony Rendon, whom he managed in the Arizona Fall League last year. (When Rizzo attended the league last season, he wasn’t only scouting players.) That was also Williams’s only experience as a manager. He will perform a job he’s never done at the highest level, or even in the minor leagues over a full season.

It is a gamble, but that does not mean Williams’s hiring is a bad bet. Those who know Williams, players who played for him and coaches who coached him, share Rizzo’s confidence in him. Red Sox third base coach Brian Butterfield coached in Arizona from 1998 through 2000, as Williams manned third base for the Diamondbacks. Even back then, he saw a future manager.

“He’s got a great baseball mind, and a great baseball temperament,” Butterfield told The Post’s Barry Svrluga at the World Series. “I think from playing and from getting into coaching, I think that that has probably — I don’t know what word to use — maybe it’s mellowed him a bit. As a player, he was tremendously intense because he wanted to hold up his end of the bargain. He got paid good money and he wanted to reward the organization, and he has a lot of pride.

“He’s one of my favorites of all time, because of how tough he is, what a great competitor he is. I’ve got a warm spot in my heart for Matty Williams. I think he will be an outstanding manager in the big leagues. I think he’s going to be able to relate to the veteran players and the young guys, and I think it’ll be reflected in the way the team plays.”

The Nationals are Rizzo’s team more than ever after he signed a contract extension and was elevated to team president, and with Williams’s hire they will be a team built more in his image than ever. Every decision that gets made passes through him, and he makes all the important ones himself. He listens to his staff and gathers information, but he is rarely swayed. Multiple people close to Rizzo believe he had his heart set on Williams months ago.

Rizzo took over the Nationals after they had lost 103 games and, with the help of high draft choices, turned them into a 98-game winner three years later. Given the team’s descent to 86 wins, how Rizzo can or can’t push the Nationals back to the 90-95 win range this season will be the best barometer yet of his ability. Is the core he built sustainable, or was 2012 only a spasm?

The makeup of the roster suggests the former, and it will be Williams’s job to ensure it happens. Rizzo went somewhat against the grain in tabbing Williams. Despite raves from people close to him, Williams had not been mentioned publicly as a serious candidate for other managerial vacancies this offseason.

Eschewing convention for his own belief fits into Rizzo’s pattern. He gave Jayson Werth a $126 million contract. He shut down Stephen Strasburg. He acts on his own instinct. Last spring, after Rizzo lauded the work of his statistical analysis staff, I asked him if he could think of a time when their work changed his mind on acquiring or not based on his scouting reports. He could not.

In hiring Williams, Rizzo undoubtedly got his man. The choice also reinforced that the direction of the franchise lies in Rizzo’s hands. Williams is his guy, and this is his franchise, and now we get to see how well it all works.