PORT ST. LUCIE — In the unhappy event star center fielder Brandon Nimmo took his ever-improving all-around game elsewhere this past winter, the Mets seemed to possess exactly one worthwhile backup plan — and it was really no plan at all.

Let’s face it, plan B — while uber intriguing, was more fantasy than reality.

The Mets’ one known apparent alternate concept was to lure superstar shortstop Trea Turner to New York to switch to center field. But while they get points for creativity for such an outside-the-box idea, one phone call likely told them they were dreaming.

“Trea didn’t like playing center field when he was in Washington,” Mets co-ace Max Scherzer, who was Turner’s teammate in D.C., told me the other day. “He never wanted to do it again.”

So while the Mets surely reached out to Turner, who to no one’s surprise signed to stay at shortstop with the rival Phillies for $300 million — while he didn’t want to change positions, the Lake Worth, Fla. product was known to prefer to come back East — they had to know their plan was basically a non-starter. Manager Buck Showalter recently and rightly identified Nimmo here as quite possibly the biggest must-have on the team’s lengthy do-to list.

Nimmo, still only 29, has overcome rare Wyoming roots — nothing against the Cowboy state, where rodeo is king — to turn himself into an outstanding leadoff hitter and center fielder. Those are reasons 1 and 1A the Mets had to have him back. The other reason is there just wasn’t any other viable choice.

Nimmo, though, had many options, from the Jays to the Rays to the Mariners to the Giants, where at least for a while, he could dream about teaming with Aaron Judge. Nimmo is a popular, adaptable guy — he’s easily transitioned from Cheyenne to Long Island City — who could envision himself many places. But he said the other day, “I definitely wanted to come back. I feel like we started something that’s going to put the Mets on a different trajectory for a while.”

Nimmo’s eventual call to re-sign, for $162 million over eight years, is one reason that’s still correct. It’s his plus personality but mostly that rare combination of skill both ways that make him indispensable. He posted an .800-plus OPS three straight years but was a 5 WAR player for the first time in 2022, thanks to vastly improved defense. When I asked a Mets executive to name the other center fielders they pursued, he couldn’t immediately recall many, if any.

The “good” choices were all long shots.

1. Cody Bellinger. Bellinger is an excellent defender and former league MVP who’s struggled offensively since an overexuberant forearm smash with Kiké Hernandez following a World Series home run and subsequent shoulder surgery. While the Cubs are banking on a return at least part way to form for Bellinger, the Mets barely reached out.

2. Starling Marte. A Marte move back to center field was untenable since Marte, 34, had surgery to repair tendons on both sides of his groin this winter.

3. Aaron Judge. Crosstown superstar Judge can play center, where he thrived in his magical, record-setting 2022 season. But they didn’t want to pursue a Yankees icon when they figured there’d be no chance he’d leave for the Mets; they needed a signal from him to even consider it, which it’s apparent they didn’t get. Many assume another reason the Mets didn’t pursue Judge is that our two team owners are enjoying an unusual era of good feeling, but it’s probably more about the Mets having so many other expensive holes to fill and not wanting to waste their time.

The Judge free agency actually had a greater effect on Nimmo, as he and Judge were being pursued simultaneously by the Giants, who wanted to add two big-time outfielders. Although the Mets were Nimmo’s first choice, the idea of teaming with Judge intrigued him.

“Just like everyone else, we were waiting to see what was going to happen with Judge because he was the big-ticket item,” Nimmo said. “Judge being from there, we thought it was possible he could end up there.”

That, of course, was before both stars ended up back where they belong. While GM Billy Eppler put his arm on Nimmo’s shoulder and told him they very much wanted him back in a clubhouse moment sometime before the All-Star break, talks didn’t start in earnest until after Judge went back to the Yankees. Both Eppler and Nimmo recalled that all came together in four short hours, when both Eppler and Steve Cohen were on the plane returning from the Winter Meetings.

Nimmo is a guy who’s as adaptable as he is affable — “I’m a when in Rome” guy,” he said — so he could envision himself elsewhere, even San Francisco (a place that has trouble drawing free agents). But he loves New York.

“Not many guys get to start their career and finish their career with one organization,” Nimmo said. “Maybe at the end you can get your number up there and be a guy who’s talked about in this organization for years to come.”

Yes indeed, ultimately, as much as the Mets needed to keep Nimmo, he wanted to stay with them, too.