Who moves on to October? What’s at stake for Mets, Braves — and D-backs?! — in Monday’s doubleheader

Yes, we have one more day of regular-season baseball to be played. Sunday’s results failed to settle the National League wild-card race, so the New York Mets jumped on a plane to Atlanta for a doubleheader against the Atlanta Braves on Monday (1 p.m. ET on ESPN2). The Arizona Diamondbacks will be watching — they need one of these teams to pull off a sweep in order to get into the playoffs themselves.

Let’s break down some of the doubleheader’s biggest questions:

OK, why are we here in the first place?

These are makeup games from the two Hurricane Helene-related rainouts in last week’s Mets-Braves series. Without them, the current standings have the Braves, Mets and Diamondbacks in a virtual tie for the final two wild-card spots, necessitating the two games be played:

Diamondbacks: 89-73
Mets: 88-72
Braves: 88-72

The important thing to know here: Both the Mets and Braves hold the tiebreaker over the Diamondbacks by virtue of winning their season series. That’s why the Diamondbacks need a sweep to get in; if the Mets and Braves split the doubleheader and all three teams finish 89-73, the Diamondbacks stay home.

Another thing to know: The Braves lead the season series over the Mets 6-5, so a split would see them remain the higher seed.

What are the scenarios for each team to clinch a playoff spot?

Let’s run through these:

1. The Mets win the first game, the Braves win the second game. The Braves are the No. 5 seed and head to the No. 4 San Diego Padres for a best-of-three wild-card series starting Tuesday. The Mets are the No. 6 seed and play the No. 3 Milwaukee Brewers. Yes, that means the Mets would have gone from Milwaukee on Sunday to Atlanta on Monday, and then back to Milwaukee on Tuesday. (The Braves would have to fly cross-country, but at least they were already in Atlanta.)

2. If the Braves win the first game and the Mets win the second, it’s the same result. The Braves are the No. 5 seed and the Mets are the No. 6 seed.

3. The Mets win both games. New York is the No. 5 seed and plays the Padres. The Diamondbacks are the No. 6 seed and play the Brewers in a rematch of last year’s wild-card series. The Mets travel from Milwaukee to Atlanta to San Diego. (They won’t complain.)

4. The Braves win both games. Atlanta is the No. 5 seed and plays the Padres; the Diamondbacks are the No. 6 seed and play the Brewers.

Keep in mind that whichever team wins the first game of the doubleheader clinches a playoff spot. That will provide less incentive — or, really, no incentive — for that team to churn through its best relievers in the second game. Not with the first wild-card game on hand for Tuesday.

Who are the starting pitchers?

Well, first of all, remember: The scheduled starter is likely to change for the team that wins the first game.

The Mets are going with right-hander Tylor Megill in the first game. Megill last pitched on Sept. 22 against the Phillies, allowing one run in four innings (but throwing 93 pitches). He has allowed just two runs over his past three starts. Right-hander Luis Severino is on the schedule for the second game. His last start was the first game of the Atlanta series last Tuesday and he took the loss, allowing seven hits and four runs in four innings. He has a 3.17 ERA over his past eight starts.

While the Mets used arguably their top three starters — Sean Manaea, Jose Quintana and David Peterson — over the weekend against the Brewers, they are all lefties. These two righties might actually be a better matchup against the righty-heavy Braves lineup that had a .778 OPS against left-handed pitchers but .706 against right-handers.

It’s worth noting here that if both scheduled starters are used and the Mets advance — if they lose the first game and win the second — they would have to use Manaea on three days’ rest to start the wild-card series.

The Braves announced that rookie Spencer Schwellenbach will start Game 1. He faced off against Severino last week and pitched a gem, allowing just three hits and one run over seven innings. He faced the Mets one other time, back in July, and had the best game of his career, striking out 11 in seven scoreless innings.

Atlanta’s starter for the second game — for now — is Chris Sale.

Wait, yeah, what’s going on with Sale?

Good question. Sale, who leads the NL in wins, ERA and strikeouts, hasn’t pitched since Sept. 19, when he went five innings against the Cincinnati Reds. He was scheduled to start one of the rained-out games, but heading into the weekend the Braves said they would now save Sale for an “emergency” situation — meaning, a must-win game. That will come into play now only if they lose the first game.

It’s certainly an interesting strategic decision — if they had won Sunday with Sale pitching, they would have clinched a playoff spot. Instead, they can use him in the second game if needed, and if they win the first game, they’ll have Sale ready to start the first game of the wild-card series.

Of course, there’s also this question: Is he 100 percent right now?

His four-seam fastball averaged 92.7 mph against the Reds — his lowest average of the season, down from 95.9 mph the start before, and down from his overall season average of 94.8 mph. Maybe it was just a little late-season fatigue: He was starting on four days of rest and his previous start had come against the Los Angeles Dodgers, when he had his second-highest average velocity. But a 2 mph drop is considered significant, so it’s certainly something to note.

Maybe there’s nothing here. Maybe the Braves were just holding Sale back with the hope that he wouldn’t be needed and thus would be rested for the start of the postseason. We’ll find out if the Braves lose that first game.

What about the bullpens?

Mets manager Carlos Mendoza used Edwin Diaz to close out the 5-0 win over the Milwaukee Brewers on Sunday. This is understandable — you don’t want to mess around in that situation. But it was a safe five-run lead and Diaz ended up throwing 26 pitches. Now, there’s a scenario where the Mets need him in six games over five days, counting Sunday’s finale, Monday’s doubleheader and the wild-card series.

The Braves are in better shape; none of their top relievers — closer Raisel Iglesias, Joe Jimenez or Pierce Johnson — pitched in Sunday’s 4-2 loss to Kansas City. They also have one of the deepest and best bullpens in the league, so they’re in good shape to withstand the rigors of a doubleheader.