Most interesting September MLB series: Yankees vs. Red Sox, Mets vs. Reds and more with playoff implications

We’ve already dipped our toes into the month of September, and that means the final month of the 2025 Major League Baseball season is underway. This means four weeks of clarifying baseball when it comes to doling out MLB’s six division titles, 12 postseason berths, home-field advantages, and first-round byes. The stakes, like the stupid and miserable temperatures of July, are high. 

So as we work our way toward playoff baseball, let’s appreciate the month of calendar ahead of us by highlighting the many series that will determine what October looks like. Just below you’ll find just that — the head-to-head clashes during September that mark the first necessary steps toward crowning a World Series champion for 2025. Let us proceed in taut and convention-defying sequential order … 

The first two of this three-game set are already in the books, but we’ll include it here just the same because the series finale is still on the docket. The Tigers are mortal locks to win the AL Central for the first time since 2014, while the Mets are angling to remain in third and final wild card position in the NL (and also maybe put some late heat on the first-place Phillies). As such, christen this one as a Theoretical World Series Preview. 

Sept. 1, 3-4: Philadelphia Phillies at Milwaukee Brewers

We move from that to a Theoretical NLCS Preview, which features the rarely-glimpsed-in-captivity Tuesday off day. Each of these NL powers has a fairly secure division lead, and the Brewers have a fairly secure grip on a first-round bye (the two division winners with the best records in each league get a bye to the LDS). The Phillies, meantime, are locked in a tight race with the Dodgers (and by extension the Padres, should they overtake L.A. at some point) for that coveted second bye on the NL side. In that sense, this one matters more to Philly. 

And here we have a Theoretical ALCS Preview that’s already underway. This marks the last time the author will note the presence of a Theoretical Playoff Series Preview. It’s also one of the most compelling junior-circuit rivalries, at least in recent years. The Astros are trying to fend off the Mariners and the Rangers in the AL West. As for the Yankees, they’re in wild card position and still have designs on catching the Blue Jays in the AL East. 

The Mets are occupying the third and final wild card spot in the NL, and the Reds are their closest challenger for that last playoff berth. The Queenslanders’ lead is a substantial one given the relatively late hour, and that means the Reds badly need to take this series and thus whittle away at the Mets’ advantage. If the Reds can take two of three or sweep, then they’ll also claim the tiebreaker over the Mets. Cincy and the Mets each play two of the toughest remaining schedules in all of MLB. 

Sept. 5-7: Texas Rangers at Houston Astros

The Rangers have been playing closer to their expected level lately, and as such they’ve returned to relevance in the AL wild card race and maybe even the AL West. Of note is that, at this writing, just the Yankees and Red Sox have a better run differential than Texas does. This series marks their first of two chances to take a direct bite out of Houston’s lead in the division. Seattle will be scoreboard watching for multiple reasons. 

Sept. 5-7: Toronto Blue Jays at New York Yankees

As noted above, the Yankees (and Red Sox) are still very much alive in the AL East race, which means this clash between the Yanks and the first-place Jays a pivotal one. Toronto owns the tiebreaker over both the Yankees and Boston, so if the AL East ends up in any kind of tie involving the Jays they’ll take the flag. That set of circumstances adds to the urgency on the part of the Yankees. 

Sept. 8-11: New York Mets at Philadelphia Phillies

There’s time for things to change, but at this juncture the Mets will need at least to take three of four in this series in order to keep the division in play. That’s of course assuming the Phillies don’t otherwise collapse before or after this series. 

Sept. 9-11: Detroit Tigers at New York Yankees

To repeat, the Tigers are playing for a bye and top seed in the AL. The Yankees are looking to hawk down the Jays in the AL East and land ahead of the rival Red Sox in the AL seedings. 

Sept. 9-11: Houston Astros at Toronto Blue Jays

Beyond the most immediate and important goal of maintaining their respective division leads, these two teams might also wind up jousting for one of the two byes available to AL teams. Houston swept their first head-to-head series of the season, so one more win gives them the tiebreaker over the Jays. 

Sept. 12-14: Kansas City Royals at Philadelphia Phillies

A strong July and August put KC back in the AL wild card race, which makes this one a series of relevance. It means more to the Royals, who are trying to catch the Mariners for that third wild card spot, but the Phillies, to repeat, are in a tight race for a bye on the NL side. 

Sept. 12-14: New York Yankees at Boston Red Sox

One of the biggest rivalries in sports carries with it playoff implications for the first time in a while. Both teams are still in the AL East race, and they occupy the top two wild card spots in the AL. That means this series might help determine who hosts that potential Wild Card Series showdown.

Sept. 12-14: Texas Rangers at New York Mets

Wild card implications on both sides with a light dusting of division-race intrigue on both sides? Yes. That’s the case with this series. Might Jacob deGrom line up to face his former team for the first time in his career? 

Sept. 15-17: Philadelphia Phillies at Los Angeles Dodgers

As promised, we will not point out that this is a Theoretical NLCS Preview. Again, it figures to be a tight race between the Phillies and the eventual NL West champ for that second bye in the NL. That’s the likely angle for Philly in this one, while the Dodgers will be playing with one eye on the second-place Padres. 

Sept. 15-17: Texas Rangers at Houston Astros

Here’s the second and final Rangers-Astros clash of September and, if trends continue, it will be relevant to both the AL West and AL wild card races. 

Sept. 16-18: Seattle Mariners at Kansas City Royals

Here’s one that could help decide the final playoff spot in the AL. The M’s hold the lead, but KC gets the head-to-head opportunity at home to whittle down that lead. The season series between the two teams is locked at 2-2, so this one will also determine who gets the tiebreaker. 

Sept. 16-18: San Diego Padres at New York Mets

The Padres are trying to catch the Dodgers in the NL West, but they have no more head-to-headers against L.A. and they lose the tiebreaker. San Diego is, however, in second wild card position in the NL with a shot at getting into the top spot and thus hosting a Wild Card Series. You’re aware of the Mets’ current straits and aspirations. Of note for this series is that the Padres are solidly below .500 in road games this season, while the Mets have been one of MLB’s best at home. 

Sept. 19-21: Seattle Mariners at Houston Astros

You know what’s up by now. The Astros hold a narrow lead over the Mariners in the AL West, and this marks their final head-to-head encounter of the regular season. The season series is also tied 5-5 going into this one, so the tiebreaker goes to whichever team takes two of three or sweeps. 

Sept. 19-21: Toronto Blue Jays at Kansas City Royals

Toronto wants its first division title since 2015 and the Royals are looking to crack the playoffs for a second straight year. 

Sept. 22-24: Milwaukee Brewers at San Diego Padres

The Padres play one of the weakest remaining schedules in all of MLB, but this series is one of the exceptions. That, of course, assumes the Brewers haven’t wrapped up the NL Central and a bye by this point and are in “rest and reset for the playoffs” mode. 

Sept. 23-25: New York Mets at Chicago Cubs

The Cubs are in top wild card position while also still hoping to pull off a minor miracle at the expense of the Brewers in the NL Central. A more realistic goal for the North Siders might be to remain in top wild card position and thus earn the right to host the Wild Card Series. By this point, we’ll likely know whether the Mets have any hope of catching the Phillies or whether their focus will have shifted to wild card matters. 

Sept. 23-25: Boston Red Sox at Toronto Blue Jays

The Red Sox have been one of MLB’s best teams since the calendar flipped to July, so it won’t be a surprise if the AL East hinges on this series (the Yankees of course may have something to say about that). 

Sept. 26-28: Detroit Tigers at Boston Red Sox 

We’ll repeat those AL East implications just noted, and the Tigers by this point might be trying to wrap up a first-round bye. 

Sept. 26-28: Los Angeles Dodgers at Seattle Mariners

Will this final series of the regular season arrive with both West divisions still in play? That’s possible given the current margins. The Dodgers have a lead over the Padres right now, while the M’s are trying to catch the Astros while remaining in wild card position. Will this mark Cal Raleigh’s final push for 60 homers?