ARLINGTON, Tex. — Someone once traded away Mookie Betts.

After the sense of awe dissipates following another spectacular play by the Los Angeles Dodgers’ superstar right fielder, after the slow-motion replays confirm Betts’s singular brilliance, after the thought is given to how outrageously fortunate the Dodgers are to have Betts in the fold as they make their march through the postseason to this week’s World Series — after all that is dealt with and put aside, the mind inevitably returns to the same stunning, inconceivable realization:

Someone once had Mookie Betts on their team — and decided they would be better off with some other players instead.

That someone, of course, is the Boston Red Sox, whom Betts, the 2018 American League MVP, helped lead to the World Series title that fall — beating, of course, the Dodgers. It is now clear, if it wasn’t at the time, that the trade eight months ago that sent Betts to the Dodgers along with pitcher David Price for a trio of younger players is the most consequential in recent baseball history — this generation’s version of Frank Robinson for Milt Pappas.

“We would have beat the Red Sox” in 2018, Dodgers Manager Dave Roberts said Sunday night, in the aftermath of their Game 7 victory over the Atlanta Braves, “if we’d had Mookie Betts.”

While the Red Sox have collapsed in on themselves — going from a championship in 2018 to a third-place finish in 2019 to last place in 2020, while undertaking an economic overhaul of which the Betts trade was the centerpiece — the Dodgers, with Betts as their indefatigable engine and franchise player, are preparing to face the Tampa Bay Rays beginning Tuesday night in Game 1 of the World Series at Globe Life Field.

The Dodgers have several, complex reasons for believing this is the most complete of their eight consecutive division-champion playoff teams, the first seven of which ended with October defeats: their enviable starting pitching depth, the many recent dividends of their player-development machine, the maturing of core players such as Corey Seager, Walker Buehler and Cody Bellinger.

But the simplest reason is this: Those teams didn’t have Mookie Betts, and this one does.

“Mookie kind of separates himself, I think, with the consistency,” said left-hander Clayton Kershaw, the Dodgers’ longtime ace. “The other things he can do on the baseball field if he happens to not be getting hits — that’s what separates him. There’s also a confidence there — just a really calming influence. Thankfully, he’s on our team.”

The Dodgers knew what they were getting in Betts when they made the trade: a dynamic ­right fielder and hitter who can alter games with his glove, his bat, his legs or his arm. The 28-year-old hit .292 with 16 homers, slugged .562 and stole 10 bases during the truncated, 60-game regular season, playing defense like the four-time Gold Glove winner he is and lifting himself into the NL MVP conversation.

But there was more to Betts than even the Dodgers realized, and that, as much as the sheer, game-changing talent, was what prompted the Dodgers to sign Betts to a 12-year, $365 million contract extension in July. A franchise that is not prone to making blockbuster moves, and that had passed on one big free agent after another over the years — all in the name of future economic flexibility — had decided, for once, to go all-in. Betts made the Dodgers change course.

“It’s incredible what he does on defense, what he does on the bases, in the batter’s box,” Dodgers president of baseball operations Andrew Friedman told reporters during a video interview. “The instincts and feel for the game are usually seen in extra players — because they have to do those extra things to get to the big leagues and stick around. You rarely see that in the best player on the field.”

“I love the coaching staff, the players, the front office,” Betts said Sunday night of his first season in L.A. “Everything about the Dodgers is winning. That’s in my DNA, so that’s why I chose to stay here.”

On Oct. 5, at the end of their workout on the eve of the division series against the San Diego Padres, the Dodgers huddled in the outfield at Globe Life Field. Two players spoke to the group. The first was third baseman Justin Turner, the Dodgers’ longest-tenured position player. The second was Betts, their newest. What Betts lacked in Dodgers tenure, he more than made up for in stature.

“We talked about it leading up to it,” Turner said of him and Betts. “Just address the team, make sure everyone is on the same page. It’s obvious we all know what’s at stake and what we’re playing for — but just [to] remind the guys not everything is always going to go our way. It might not always be easy. But as long as we keep mentally grinding, support each other and play together as a group, we’ll get through anything.”

Betts backed up his words with his play. He went 3 for 7 with three doubles and drove in three of the Dodgers’ seven runs in a two-game mini-sweep of the Milwaukee Brewers in the first round, then went 4 for 12 with two more doubles in a three-game sweep of the Padres.

Against the Braves in the NLCS, Betts hit just .269, slugged just .308, had just one extra-base hit and drove in just one run. But that hardly conveys how thoroughly and definitively he tilted the series in the Dodgers’ favor after they had fallen behind three games to one.

On each of the next three nights, beginning with Game 5, Betts made a play in right field that saved runs and altered the course of the game.

The most baseball-savvy of them came in Game 5 — Dodgers down 2-0 in the third inning, runners on second and third, one out — on a sinking liner hit in front of him. Betts immediately knew the only chance he had to throw out the runner tagging from third was to remain on his feet instead of diving. His sprinting, reaching, shoestring catch leading to a double play saved the game for the Dodgers and sparked a comeback win that kept their season alive.

The most athletic play came in Game 6 — Dodgers up by three in the fifth inning, runner on first, two outs — when he made a sprinting, leaping catch of Marcell Ozuna’s deep drive on the warning track, saving another run and closing the door on a Braves comeback.

But Betts’s most memorable play, and the one he would later cite as his favorite, came in Game 7 — when he leaped at the wall to steal a home run from Braves first baseman Freddie Freeman.

“The one [in Game 6] was probably the most important. I think we stopped some momentum there for sure,” Betts said after Game 7. “But I think today was my favorite, since it was actually a home run.”

In the aftermath of the Game 7 play, Red Sox fans, full of the accumulated angst of watching their former franchise player performing his familiar heroics for the Dodgers, vented so loudly online that “Red Sox” for a time was trending on Twitter in the middle of the Dodgers/Braves finale.

Organizations spend millions of dollars, scouts sort through thousands of prospects, and front offices devote hours and days and months and years to the mission of locating, nurturing and deploying a player such as Mookie Betts. If you’re lucky, you find one like him in a lifetime.

The Dodgers have Betts on their side. It is good to be them — especially now, entering Game 1 of the World Series. But someone else had him and then sent him away. Woe be to them.