The Nationals pitched 36 innings in the National League Championship Series -- good ones, allowing only five earned runs in what was arguably the most dominant NLCS or ALCS performance of all time -- and 33 of them, or nearly 92%, came from just six pitchers: starters Stephen Strasburg, Aníbal Sánchez, Max Scherzer and Patrick Corbin, plus relievers Daniel Hudson and Sean Doolittle.

Javy Guerra, one of the 11 pitchers on the NLCS roster, didn't pitch. Austin Voth didn't pitch. Roenis Elías didn't pitch. Wander Suero, who was on the roster only for Game 1 while Hudson was on paternity leave, didn't pitch. Veteran Fernando Rodney actually did get in, but he threw all of nine pitches across two games. It was similar in the NLDS against the Dodgers, where The Big Six threw 87% of the innings.

That's fine, perhaps, in a five-game series, as the NLDS was, giving manager Dave Martinez the chance to bring in Corbin twice in relief and Scherzer once. It worked out well enough in a four-game sweep in the NLCS, where no starter had to start twice. But in the World Series, against a much higher caliber of opponent than they faced in the NLCS, four starters and two relievers aren't going to be enough over potentially seven games. There's got to be someone else who can appear in high-leverage situations. It won't be Suero, or Guerra, or Voth.

Enter 26-year-old rookie Tanner Rainey, the most unexpected of secret weapons -- and one who is almost guaranteed to be relied upon for some important plate appearances against the Astros.

Now, you may not know who Rainey is or why he matters, and that's perfectly understandable. We'll catch you up on the biography. But really, there's three things you need to know:

1) He throws really, really, really hard;

2) His slider is the best pitch you don't know; and

3) Just look at what he did to a pair of veteran Cardinals hitters

Let's start from the bottom, first, so you can see why Rainey is worthy of such examination. In Game 3 of the NLCS, Rainey faced Cardinals catcher Yadier Molina in the ninth inning. Molina saw five fastballs, ranging from 98.4 to 100.3 mph, watching three and fouling off two. On the sixth pitch, Rainey dropped an 89.9-mph slider diving out of the zone. It ended poorly for Molina.

The next night, in Game 4, Rainey faced Cardinals shortstop Paul DeJong. The first four pitches were fastballs, ranging from 98.8 to 100.5 mph. On the fifth pitch, Rainey dropped an 89.1-mph slider diving out of the zone. It ended poorly for DeJong.

A predictable pattern? Perhaps, but that's a rookie pitcher flashing triple-digit heat and making two hitters with All-Star selections on their resumes look pretty silly in some big postseason spots.

Since it's very likely you're not terribly familiar with Rainey, let's take a brief minute to catch you up. A second-round pick of the Reds in 2015 out of the University of West Alabama, Rainey spent four seasons in the Cincinnati Minor League system both impressing with his velocity and ability to miss bats -- 339 whiffs in 275 1/3 innings -- while causing worry if he could ever throw enough strikes for it to matter, since he walked a far-too-high 162 batters, nearly 14% of those he faced. He got into seven innings in the Majors for Cincinnati, then in December, Rainey was the return when the Nationals traded Tanner Roark to the Reds in advance of signing Sánchez.

Rainey didn't make the Opening Day roster -- it's wild to look back now and see that the long-departed Trevor Rosenthal, Kyle Barraclough, Matt Grace, Tony Sipp and Justin Miller joined Doolittle and Suero -- and after spending the first third of the season with Triple-A Fresno, he joined the Nats on May 18.

Once he arrived, he stuck for the rest of the season, posting elite strikeout rates -- 34.6% of batters faced, 21st of the 398 pitchers who threw 50 innings -- while continuing to post worrisome walk numbers, with his 17.8% mark being 398th of those 398, which is to say, the highest.

"He’s got a live fastball," Martinez said the day Rainey was recalled. "His slider toward the end of Spring Training was really, really good.”

Well, yeah. That's a pretty accurate scouting report. Let's explain what Martinez meant by his "live fastball" and "really, really good slider."

Rainey's fastball velocity is almost unmatched

Obviously, because you just watched him hit 101 in those clips above. Counting the playoffs, Rainey is one of just eight pitchers to hit triple-digits at least 20 times this year. His average of 97.7 MPH is a Top 20 mark, putting him in the 98th percentile. Very few pitchers throw this hard, and because he's got above-average spin, he's also got above-average movement, too, adding 2.1 inches of rise above the average at his velocity.

But for all that heat, his fastball wasn't exactly dominating, allowing a .455 slugging percentage and 10 extra-base hits. If it did anything, it was to set up that "really really good slider," which, in fact, was.

Rainey's slider was an elite pitch

“I’ve been able to use my slider, both to get into counts and put guys away, and that’s been beneficial for me,” Rainey told a local broadcast after a game on June 2.

“Obviously the fastball is not always exactly where I need it to be, but being able to get into the counts with the slider ... and also having that to put someone away with makes the fastball play a little better.”

Perhaps because the fastball demanded such respect, the slider played up, and if that sounds more than a little like what Aroldis Chapman has done, it should. The slider:

Had the highest swing-and-miss rate of any slider in baseball.

There were 222 pitchers who got 100 swings on their slider, and no one had a higher rate of misses than Rainey's 63.1%. That's impressive, but it also undersells it, believe it or not. Let's go back to the dawn of the pitch tracking era in 2008, find the thousands of pitcher seasons with at least 100 swings on a slider, and we'll find that Rainey just had one of the best slider swing-and-miss years on record:

71.5% -- Jonny Venters, 2011
65.9% -- Jonny Venters, 2010
65.5% -- Sergio Santos, 2011
63.1% -- Rainey, 2019 <------------
62.0% -- Ken Giles, 2016

Had Top 5 slider outcomes of any slider in baseball.

As you'd expect, missing that many bats makes your pitch a pretty productive one. The way we can look at that is to check out the 231 pitchers who had at least 50 plate appearances end on a slider, and rank them by lowest Weighted On-Base Average allowed. Rainey is in the Top 5.

.131 -- Yency Almonte, Rockies
.140 -- Carlos Martinez, Cardinals
.143 -- Ken Giles, Blue Jays
.149 -- Tanner Rainey, Nationals <------------
.151 -- Jeff Brigham, Marlins

Has top 10 vertical drop among sliders in baseball

You saw how north to south those pitches that got DeJong and Molina were, right? That's because he gets strong drop on his slider, an extra 5.3 inches of drop over other sliders near his velocity and release point. That's an elite number -- it's 10th best of those who have thrown at least 250 sliders.

Now, all of this only goes to speak to the obvious talent Rainey possesses, and how we've already seen flashes of it at the highest level. It doesn't position him as a star, because as we said, no one had a higher walk rate in baseball this year, which is a somewhat terrifying proposition should he be asked to face José Altuve, Alex Bregman, or George Springer in a big spot. (Rainey has a .139/.298/.228 line against righties, but was hit hard by lefties, .261/.420/.522, in part because he doesn't have a usable third pitch right now.)

The walks have been a problem, and the walks will be a problem. But the arm is for real. The fastball velocity is for real -- when he can throw it for strikes -- and the slider has all the signs of an elite pitch. Due to the limitations of the rest of the Washington bullpen, Rainey might be forced into situations he wouldn't otherwise see on a deeper staff. If he can simply get it over the plate, as he did against St. Louis, he could be a secret weapon for Martinez to call upon. He could be the one on the mound in the biggest situations.

After all, it can't always be Doolittle, or Hudson, or a starter. It shouldn't be Suero or Voth or Guerra. Tanner Rainey, it's your time to shine.