Buddy Harrelson waited anxiously in the offices of Dr. Max Rudansky, a Huntington-based neurologist who had asked the Mets legend for the list of troubling symptoms.

It was the summer of 2016 and words seemed to be disappearing from Harrelson’s vocabulary. He had difficulty finishing sentences and completing thoughts and often lost his place in a conversation. These weren’t new concerns: In 2013 the family had taken Harrelson to a different doctor, who attributed the decline to natural aging, stress and possibly depression.

But the red flags kept appearing. Harrelson started getting lost driving familiar routes. His ex-wife, Kim, was a car-length behind when Harrelson zoomed past a turn he’d taken a million times in his Hauppauge neighborhood. He then over-corrected by taking a sharp left from the far-right lane, nearly causing an accident.

Rudansky listened as Kim described the harrowing experience before Harrelson admitted this wasn’t the first time he’d become disoriented. In October 2015, Harrelson was driving to his condo in Venice, Fla., where he visited after every baseball season. It was an annual trek — he knew the roads by heart.

Except this time.

“I had no idea where I was,” Harrelson said. “Scared the hell out of me.”

All the data pointed in the same direction. The doctor, armed with test results from an MRI and CT scan, plus his own observations during the office visit, exhaled slowly before delivering the diagnosis.

He looked at Kim and said, “[Harrelson] has Alzheimer’s.”

It was at that moment both Buddy and Kim started crying.

That was the beginning of Harrelson’s journey, which will go in only one direction. Alzheimer’s, which afflicts approximately 5.5 million Americans and 44 million people worldwide, has no known cure. Those figures are expected to double every 20 years unless science finds a way to stop the buildup of tau, proteins that damage brain cells.

It starts with memory loss and confusion. In time, the disease progresses to complete cognitive impairment with the shutdown of bodily functions. The timetable varies for each patient, though the 73-year-old Harrelson’s case is still considered mild to moderate. His personality is mostly intact and he’s fully aware of what’s happening to him — and where this is headed.

Yet the tough little shortstop from the Mets’ 1969 championship team has moved past the shock of Rudansky’s diagnosis. In addressing his illness for the first time during a 90-minute exclusive interview with The Post, Harrelson bravely said: “I’m not afraid. I see a lot of good people who have this, just like me. It’s OK; we’re not alone.”

That can-do attitude will be needed in the coming years. It was one thing to stand up to Pete Rose in the famous 1973 NLCS brawl, but quite another to communicate when words keep disappearing, when you’re no longer allowed to drive or answer fan mail because signing an autograph requires motor skills that have vanished. Or when a simple task such as tying one’s shoes is suddenly a challenge.

That was a threshold Harrelson sadly crossed just the other day. His daughter, Kassie, had to help with the laces — Buddy had simply forgotten how to do it. And it’s more than coordination that is eroding. It takes only a few moments of chatting to realize Harrelson’s once-chipper personality is slowly flickering out.

It’s not that Buddy can’t converse. It’s just that the back-and-forth, even with friends, evaporates after a sentence or two. When he speaks, the words are halting and unsure.

“It used to be, you’d ask him a question and you could sit back and listen for the next 20 minutes,” said his 47-year-old son, Tim. “My dad would just keep talking, he was the life of the party. But now, if I don’t say something to keep the conversation going, he just gets quiet.”

Helping with word retrieval is also incumbent on the other party. As Harrelson spoke about Tom Seaver, his best friend among the back-in-the-day Mets, the conversation turned to last year’s California wildfires that consumed over a million acres.

Seaver and his wife, Nancy, were forced to flee their home in Calistoga at 4 a.m. on Oct. 27, narrowly escaping the flames. Harrelson expressed anxiety at the thought of Seaver running for his life, though the language deficit stopped him.

“Tom had to go away …”

Pause. A wave of the hand.

“It was going towards him …”

Pause.

“The … burning. The … burning was coming.”

Finally, Tim Harrelson interjected, “You mean the fires, Dad.”

Buddy nodded, looking relieved.

“The fires.”

The sad irony is that Seaver is also suffering from cognitive issues, though he attributes it to Lyme disease. But the two men have no problem remembering each other: Like most Alzheimer’s patients, Harrelson’s long-term memory is almost entirely intact. His recall of the ’69 season is excellent, as are his recollections about Tom Terrific.

Getting punched out by Rose is still a favorite talking point, too, though Harrelson has long since forgiven his one-time enemy.

“I love that guy,” he said of Rose. “He could beat you in so many ways, a really smart player, one of the best in the game. He signed a picture [of the fight] and wrote, ‘Thank you, Buddy. You made me famous.’”

And even the less glamorous aspects of Harrelson’s career — managing the Mets as they declined in the early ’90s — needed little prompting. Harrelson, who was one of Davey Johnson’s coaches throughout the ’80s, took over on May 30, 1990, after Johnson was fired. Harrelson himself was dismissed late in 1991 after compiling a 145-129 record, missing the playoffs both years.

“I didn’t ask to manage, I always wanted to be a coach,” Harrelson said. “It was tough for me, I did what I could do. I didn’t even feel like I was the manager; I was just a guy. It wasn’t a good team.”

We spoke from within our conjoined time tunnel. I covered the Mets for the Daily News when Buddy was managing. I was at The Post when he was coaching. He remembered me the moment I walked through the door of his Hauppauge home. Yet, when I asked if he’d heard from Davey, Harrelson’s face went blank.

“Davey who?”

Tim explained that his father isn’t particularly close to Johnson. Since the two rarely speak, it’s not surprising that Davey’s face and name have been erased by Alzheimer’s.

But there are members of the Mets family aware of Harrelson’s condition, including Ron Darling, who remembers Buddy as a “friend and mentor to me.”

“I’ve heard about what’s going on with Buddy,” Darling said by telephone this week. “All I can tell you is that it was easy to see why the ’69 guys loved him. He was great on defense and he was tough.

“As a coach, Buddy was the one who taught me how to bunt. He was the one who taught me how to field my position. And as a manager, he was tougher than you’d think. You could go to a certain point with Buddy, but you didn’t [mess] with him all the way. You knew not to push him.”

But Darling fast-forwarded to the present day, asking for the Harrelsons’ contact information so he could make the drive to Long Island before spring training.

“I need to see Buddy,” Darling said.

What he’ll find is a strong support system. Two of Harrelson’s five children, Kassie and Troy, live in the house. Tim, the eldest son, is 20 minutes away. Kim, his ex-wife, is also nearby. They call themselves Team Harrelson as they hunker down, hoping to buy time as doctors seek a remedy.

They’re responsible for Buddy’s transportation; his BMW convertible was put up for sale immediately after the diagnosis. They make sure the house is clean and that he eats properly. They leave Post-it notes everywhere, with little reminders like “Don’t let [Kassie’s] dogs out” or else they might run away.

Mostly, the family makes sure Harrelson stays engaged with the outside world. As scientists experiment with one drug after another, old-fashioned social interaction remains one of the best weapons.

“Talking to people makes a difference,” Tim said, which explains why Buddy and Kim remain in contact with the Alzheimer’s Foundation in Manhattan, and why they make weekly trips to the local library for the Alzheimer’s Association workshops.

Folks around town know and love Buddy, aware of the long road ahead. That’s especially true when Harrelson appears at the home games of the independent Long Island Ducks. He’s been a coach, manager and is still part-owner. These days his job description is goodwill ambassador for what he calls “the best thing I’ve ever done in baseball.”

The thinly veiled swipe at the Mets was indeed intentional. Harrelson said he rarely hears from the Wilpons and instead has shifted his loyalty to the Ducks. There, at Bethpage Ballpark, he can be himself and not be self-conscious about his speech or memory lapses.

“I feel like I’m home when I’m there, I’m with the people I love,” said Harrelson, who works the stands like a celebrity greeter. It’s good for the club and even better for Buddy as he soaks in the affection.

“I want people to know you can live with [Alzheimer’s] and that a lot of people have it,” Harrelson said. “It could be worse.”

Indeed, and though Kim remembers her tears in Rudansky’s office and being overwhelmed by the “devastating” sense of doom, she and the rest of Team Harrelson are moving courageously ahead.

“If Buddy coming forward can help one person or one family with Alzheimer’s, then that’s his goal,” she said. “That’s why he’s coming forward. To help others.”

Award-winning baseball writer Bob Klapisch covered the Mets and Yankees for The Post from 1983 to 1987. He has also written for the Daily News and Bergen Record. The author of five books, Klapisch has been a voting member of the Hall of Fame since 1993.