The Astros lost Charlie Morton and Dallas Keuchel to free agency after 2018, and Gerrit Cole left Houston after ’19. Zack Greinke, Lance McCullers Jr. and Justin Verlander, who might miss all of 2021 following Tommy John surgery, can test the open market next offseason. And Forrest Whitley, part of MLB Pipeline's Top 100 prospects list five years running, still hasn’t reached the Majors.

As a result, the future of the Astros' rotation is mired in uncertainty. But Houston may have found its next ace in 2020.

With Cole gone and Verlander sidelined for all but one start, it wasn’t Greinke or McCullers who led the Astros in innings and strikeouts last season. It was Framber Valdez.

After posting a 5.86 ERA and a 1.67 WHIP in 2019, Valdez turned in a 3.57 ERA with a 1.12 WHIP over 70 2/3 innings in the ’20 regular season, then added four stellar starts covering 24 innings in the postseason.

All told, Valdez threw more innings than any other pitcher in the Majors in 2020, firing 94 2/3 frames and recording a 3.14 ERA with 102 K’s and 26 walks.

Houston also got strong contributions from 23-year-old Cristian Javier and 25-year-old Jose Urquidy last season, but Valdez has the best chance of keeping it up in 2021 and beyond. Here’s why the 27-year-old lefty could become a mainstay atop the Astros’ rotation.

His curveball is one of MLB’s most dominant pitches

In terms of movement, Valdez's curveball is a rarity. The pitch, which had an average spin rate of 2,982 rpm, got 5.8 inches more vertical drop and 4.4 inches more horizontal break than average curves thrown with similar velocity in 2020. That was tied for the 23rd most added drop and the 24th most added break among qualifiers last year.

He was one of eight pitchers who had a curveball that ranked in the top 50 in both metrics, and only one other -- Valdez’s teammate Ryan Pressly -- threw a curve that was top 25 in both.

Additionally, Valdez’s curveball is a mirror image of both his sinker and changeup coming out of his hand. More precisely, his curveball spins on the same axis as his sinker and changeup but in the exact opposite direction. This can make it very difficult for hitters to tell the seams apart, as MLB.com’s Mike Petriello noted when he broke down Cleveland right-hander Shane Bieber in January, and as this GIF from Michael Augustine shows.

This video from Rob Friedman -- aka the "Pitching Ninja" -- illustrates how Valdez is able to effectively use his sinker and curveball in tandem to keep hitters off balance.

Considering all of that, it’s really no surprise that Valdez's hook has consistently baffled big league hitters.

In his three-year career, the lefty has ended 266 plate appearances with his curveball and recorded 137 strikeouts with the pitch. His 51.5% strikeout rate on curveballs was the third highest for any specific pitch type in the Majors across 2018-20 (minimum 250 plate appearances).

His curveball has yielded a .117 average and a .222 slugging percentage, and his .192 xwOBA -- based on strikeouts, walks and quality of contact -- allowed on the pitch ranks among the best in the game since the beginning of 2018.

Lowest xwOBA allowed on a specific pitch type since 2018
Minimum 250 plate appearances ended with that pitch

1. Germán Márquez’s knuckle curve: .176
2. Blake Snell’s curveball: .183
3. Trevor Bauer’s slider: .185
4. Dylan Bundy’s slider: .187
5. Framber Valdez’s curveball: .192

Valdez leaned on his curveball more as 2020 went on. After throwing it 31.6% of the time in his first eight games, he increased his usage rate to 42.0% over his final seven outings (postseason included) and posted a 2.55 ERA with a 32.0% strikeout rate in that stretch.

Seven of the 11 highest single-game curveball usage rates of his career came in 2020, including No. 1: Game 6 of the American League Championship Series against the Rays, when he went to the pitch 51.5% of the time and held Tampa Bay to one run on three hits with nine strikeouts in six innings.

He’s a ground-ball machine

Every pitch Valdez uses regularly has above average vertical movement, making it difficult for hitters to get under the ball.

A whopping 51.6% of the batted balls hit against him over the past three years have been classified either as “poorly/topped,” the highest in MLB, and he’s the only pitcher (minimum 400 batted balls allowed) to post a negative average launch angle in that span.

Valdez’s fifth-percentile average exit velocity (91.4 mph) and fourth-percentile hard-hit rate (48.7%) in 2020 might seem troubling, but they become less so when looked at with some added context.

While Valdez's overall hard-hit rate was the third highest among pitchers who allowed at least 100 batted balls last season, his 23.0% hard-hit FB+LD rate was middle of the pack.

In an era when home run rates have skyrocketed, being able to avoid air contact consistently is an incredibly valuable skill. Just 14 of the 288 plate appearances against Valdez in 2020 ended in an extra-base hit, one of the lowest rates in MLB.

Lowest XBH% among qualified pitchers in 2020

1. Dallas Keuchel: 4.3%
2. Shane Bieber: 4.4%
3. Framber Valdez: 4.9%
4. (tied) Dinelson Lamet: 5.2%
5. (tied) Zac Gallen: 5.2%

Walks down (and strikeouts up)

One of the biggest reasons behind Valdez’s 2020 success was that he stopped handing out so many free passes. His walk rate last season was just 5.6%, which ranked in the 86th percentile.

It was a surprising display of control from a pitcher who walked 68 of the 483 batters he faced over 2018-19, recording the third-highest walk rate (14.1%) among pitchers who threw at least 100 innings in that span.

How’d he do it? In addition to pitching with the count in his favor more often in 2020, he was better at bearing down when he got into three-ball counts.

Across 2018-19, his strike-zone rate on three-ball pitches was just 55.1%, and he often made it an easy decision for hitters to lay off his pitches, putting more than 29.2% of his three-ball offerings in either the “chase” or “waste” zones.

He was much more aggressive in these counts in 2020, posting a 64.3 strike-zone rate and throwing only 21.4% of his pitches in the “chase” or “waste” zones when he got to three balls.

This new approach paid off, as Valdez yielded a .376 xwOBA in three-ball counts in 2020, compared to a .519 xwOBA in ’18-19. He battled back to record a strikeout 17 times after getting into a three-ball count last season, the same number he had the previous two years combined.

Overall, he recorded a 26.4% strikeout rate in 2020, adding 5.3 percentage points to his mark (21.1%) from 2018-19.

This gave the southpaw a rare trifecta in 2020. Since Statcast began tracking in 2015, only nine starting pitchers, including Valdez, have faced at least 100 batters and recorded a strikeout rate of 25.0% or better, a walk rate of 6.0% or lower and a ground-ball rate of 50% or higher in a single season. Here's the list:

Aaron Nola: 2016
Carlos Carrasco: 2015
Clayton Kershaw: 2015, ’16, ’20
Framber Valdez: 2020
Frankie Montas: 2019
Jake Arrieta: 2015
Masahiro Tanaka: 2017
Nathan Eovaldi: 2020
Noah Syndergaard: 2016

Kershaw was the only one who was able to pull it off multiple times in that stretch, though Nola, Carrasco and Syndergaard came close in other years. In any case, it's a good list to be on, as most of those pitchers have enjoyed sustained success in the Majors.

With continued reliance on his brilliant curveball, Valdez could get to that level as well, and give the Astros their next frontline starter.