Reds right-hander Luis Castillo has tantalized with his stuff since he debuted in 2017, but he’s never managed to break into the upper echelon of ace starting pitchers, creating a complicated dilemma for contenders in need of rotation help as the Trade Deadline approaches.

The trade market isn’t exactly brimming with elite starting pitching options, and Cincinnati’s asking price for Castillo may only go up after the A's Frankie Montas and teammate Tyler Mahle suffered shoulder injuries this week. With Montas and Mahle ailing, there arguably isn't a close second to Castillo among the best available starters, unless the Marlins decide to trade Pablo López.

Slated to start Friday against the Rays, Castillo has recorded a 3.09 ERA with a 9.3 K/9 over 11 starts this season and owns a lifetime 125 ERA+ across 134 starts in the Majors.

Last summer, the Twins got back two MLB Pipeline Top 100 prospects from the Blue Jays for José Berríos, who had an extra year of team control remaining before free agency at the time of the trade, as Castillo does now. It may take even more to get Castillo, considering the starting pitching market isn’t as strong in 2022.

While that might seem excessive, here are four reasons why the 29-year-old is worth the cost. (All stats are through Wednesday.)

1) His four-seamer performance is finally matching his velocity readings

Castillo has ranked among the hardest-throwing starting pitchers in the game since his 2017 debut, regularly sitting around 97 mph and occasionally hitting triple-digits. However, he’s struggled to find consistent success with his four-seamer -- until this season, that is.

Opponents have looked overmatched against the pitch in 2022. Castillo’s four-seam fastball whiff rate (misses / swings) is the highest in MLB by a considerable margin, and no starting pitcher (min. 100 swings) is within eight percentage points of the Reds hurler.

Highest four-seam fastball whiff rate, SP, 2022
Min. 100 swings on four-seamers

1. Luis Castillo (CIN): 41.8%
2. Eric Lauer (MIL): 33.3%
3. Gerrit Cole (NYY): 31.6%
4. Zack Wheeler (PHI): 31.4%
5. Chris Bassitt (NYM): 30.6%

Prior to this season, Castillo’s highest whiff rate on four-seamers was 37.2% during the shortened 2020 season. His highest in a full season was 30.5%, which he accomplished last year.

Unsurprisingly, the sizable increase in missed bats has coincided with a dramatic improvement in his four-seam fastball effectiveness.

Castillo’s four-seamer performance:

• 2017-21: .263 BA / .515 SLG
• 2022: .153 BA / .237 SLG

Castillo has finished off 31 of his 67 plate appearances ending on four-seamers with a strikeout, generating a 46.3% strikeout rate that also tops the MLB leaderboard. His K-rate on four-seamers from 2017-21 was 21.8%.

Highest four-seam fastball strikeout rate, 2022
Min. 50 PAs ending on four-seamers

1. Luis Castillo (CIN): 46.3%
2. Chris Bassitt (NYM): 43.5%
3. Devin Williams (MIL): 43.4%
4. (tie) Mychal Givens (CHC) and Eli Morgan (CLE): 40.0%

But Castillo's four-seamer improvement hasn't come as a result of a velocity bump or a change in location. Rather, the biggest difference has been ...

2) He’s learning to mix his pitches better

... his pitch sequencing.

Like many pitchers, Castillo typically has used the fastball to set up his offspeed and breaking pitches, but that approach has allowed hitters to hunt for heaters early in counts.

From 2017-21, Castillo went to his four-seam fastball far more often than any other offering on the first or second pitch of a plate appearance. Batters pummeled it, hitting .333 with a .683 slugging percentage on first- or second-pitch four-seamers.

In 2022, he has revamped his early-count approach.

Castillo’s combined pitch usage rates in 0-0, 0-1 and 1-0 counts

2017-21:

• Four-seamer: 36.1%
• Sinker: 23.8%
• Slider: 21.5%
• Changeup: 18.5%

2022:

• Slider: 29.2%
• Sinker: 25.5%
• Four-seamer: 24.7%
• Changeup: 20.6%

In turn, a much higher percentage of his four-seamers are now coming in two-strike counts, when batters have to be more protective instead of aggressive. Of the 320 four-seamers he's thrown this season, 35% have come with two strikes, compared to 25.4% from 2017-21.

On the whole, it's become much more difficult for batters to guess what's coming when they face the Reds pitcher. Among hurlers who have thrown at least 200 pitches this season, Castillo and the Marlins' Sandy Alcantara are the only two who have a usage rate of 22% or higher on four different pitch types.

Of course, this only works because ...

3) He has two plus secondary pitches

… his changeup and slider are so good.

The changeup has been Castillo’s best pitch since he joined the Reds. Averaging 88.2 mph with 32.4 inches of drop and 17 inches of arm-side run this season, the pitch has made countless batters look foolish over the years.

Castillo has thrown his changeup nearly as much as his four-seamer during his career, and opponents hit .176 with a .278 slugging percentage, a 41.2% whiff rate and a 40% K-rate against it from 2017-21.

The righty’s whiff and strikeout rates on changeups this season are actually the lowest of his career, in part because he's using it more to bail himself out when he gets into less favorable counts. However, batters still aren’t doing much damage against the pitch, hitting .189 and slugging .324.

Meanwhile, Castillo’s slider has become an increasingly important offering for him. As we noted earlier, he's gone to the slider more than any other pitch in early counts in 2022, and his overall usage rate has risen to a career-high 22.1%.

Opponents have posted a .178 average and a .289 slugging percentage against his slider this year, in line with his career numbers (.183 BA, .330 SLG).

Having allowed a .153 average on four-seamers, a .178 average on sliders and a .189 average on changeups, Castillo is one of five pitchers who have held hitters to a sub-.200 batting average on three different pitch types (min. 40 PAs ending on each pitch type) this season.

It’s a star-studded list.

Pitchers with a sub-.200 BA allowed on three different pitch types, 2022
Min. 40 PAs ending on each pitch type

• Sandy Alcantara: changeup, slider, four-seamer
• Corbin Burnes: curveball, slider, cutter
• Luis Castillo, four-seamer, slider, changeup
• Max Fried: slider, changeup, curveball
• Justin Verlander: curveball, slider, four-seamer

4) He keeps the ball in the park

Another reason Castillo is an intriguing trade target, even at the price he's likely to command, is because he's spent his entire career in one of the toughest environments an MLB pitcher can face, yet he's consistently been one of baseball's best at suppressing the long ball.

Since the beginning of 2020, Great American Ball Park has the highest home run park factor in MLB at 156.

In that same timespan, Castillo has served up just 29 dingers in 1,362 plate appearances (2.1%), with 13 coming at home.

Castillo's experience in Cincinnati provides a level of comfortability for potential suitors, who don't have to worry about how he'll handle a new home park.

Castillo isn't perfect, and there's likely going to be some sticker shock when teams engage in trade talks with the Reds. But he has a chance to make a real difference for a contender down the stretch and in the postseason.

Is he a top 10 starter? No. But he might be closer than you think.