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Major League Baseball's 2025 postseason will proceed on Sunday with the start of the league championship round. The American League's matchup will pit the Seattle Mariners against the Toronto Blue Jays and will get underway tonight. Over in the National League, which will start on Monday, the Los Angeles Dodgers will take on the Milwaukee Brewers.

The respective winners will advance to face each other in the 2025 World Series.

As is tradition, CBS Sports brings in the start of every new playoff round by highlighting a plausible X-factor in each series that could make the difference. There is no real definition of an "X-factor" beyond it being whatever could play a role in the proceedings. Sometimes, that's a piece of insight gleaned from granular analysis; other times, it's almost glaringly obvious. Keep that in mind, and we'll be doing just fine.

Now, let's get to why you clicked in the first place.

ALCS: Mariners vs. Blue Jays

  • X-factor: Splitter performance

The Blue Jays had four different pitchers throw at least 25% splitters during their series against the New York Yankees: starters Kevin Gausman (45.3%) and Trey Yesavage (37.2%) and relievers Jeff Hoffman (34.9%) and Seranthony Domínguez (26.9%). It's a small sample size, to be certain, but it's perhaps notable that each of those usage rates were higher than their regular-season marks.

That shift made sense. The Yankees slotted into the middle of the pack in performance against splitters during the regular season, posting a .572 OPS. Why is any of this relevant? Because the Mariners were even worse against splitters than the Yankees did, ranking 22nd in OPS (.565). For context: the only playoff team to perform worse against splitters was the Cincinnati Reds.

Seattle's best hitters, switch-hitting backstop Cal Raleigh and center fielder Julio Rodríguez, were still top performers when they faced a splitter during the regular season, posting OPS above 1.000. Otherwise? Not a single other Mariners batter who was part of their ALDS roster finished with an OPS higher than .670 against splitters. Several important supporting cast members -- including outfielder Randy Arozarena, first baseman Josh Naylor, and infielder/DH Jorge Polanco -- actually finished below .500.

There are no guarantees that the Blue Jays will spam the Mariners with splitters, but c'mon, it seems like an obvious strategy. It's on Seattle to adjust and adapt. Otherwise, the Blue Jays would seem to have a clear pathway to success in games started by either Gausman and Yesavage. That may prove to be enough for them to win and advance to the Fall Classic.

NLCS: Dodgers vs. Brewers

  • X-factor: Bullpens

When it comes to the bullpens, this is a battle of contrasts in terms of quality and quantity. 

During the regular season, the Brewers ranked sixth in unit ERA; the Dodgers checked in at 21st. Moreover, FanGraphs tracks this statistic called "meltdowns," which uses Win Probability Added to gauge how much a player contributed to (or subtracted from) their team's win expectancy. Milwaukee's relievers collectively had the 11th-fewest meltdowns in the majors. The Dodgers, conversely, had the second most, behind only the lowly Chicago White Sox. 

This isn't quite the same Dodgers bullpen now that some of their surplus starters, like Emmet Sheehan and Roki Sasaki, are working in relief. Still, the Brewers would seem to have the edge here.

Correspondingly, the Dodgers have taken more steps to prevent their bullpen from being overexposed. They've thrown fewer innings this postseason than the Brewers, with that despite them having played more games. Five of Los Angeles' six starts this postseason have lasted at least six innings. Comparatively, the Brewers haven't had a single pitcher throw six innings in an appearance this month -- not as a starter, not as a reliever. Only Freddy Peralta has exceeded five innings, and he did it once.

These are the only two NL teams left standing. Clearly there's more than one way to get through nine innings. But this is going to be an intriguing battle of contrasts: the club with the leaky bullpen that attempts to leverage their great rotation for all it can muster, even if it means turning over the lineup more often than they might like, versus the team that cannot wait to change pitchers and avoid the times-through penalty.

How those relievers perform when they are called upon by either side will by and large determine who takes home the pennant.