Luis Castillo vs. George Kirby: The Battle to be Seattle's ace

Who is it going to be? Luis Castillo or George Kirby? It's a really fun problem to argue about as we try and determine who is going to be the team's ace.
Washington Nationals v Seattle Mariners
Washington Nationals v Seattle Mariners / Steph Chambers/GettyImages
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The Seattle Mariners are suffering from the success of having two Cy Young candidates at the top of their rotation. With the duo of Luis Castillo and George Kirby, there is no better one-two punch in the league. 

Pitching coach Pete Woodworth outlined the difference between the two pitchers in their mentalities and approaches to their starts, with Castillo laid back and Kirby constantly excited.

"Mound visits with Luis are always a blast. Mound visits with George are not fun. He doesn't want me to be out there for one more second. So yeah, they are different on game day."

Pete Woodworth

Both generate their success in different ways, but they both have been extremely successful in 2023. Fighting for the AL West crown and looking for a deep playoff run, there remains the question of who gets the ball if they need a win, Castillo or Kirby, or simply put, who is the ace of this staff?

The case for Luis Castillo

Since coming over from the Cincinnati Reds last year, he has been nothing short of dominant. The 2023 season has been no different, sitting at 11-7 with a 3.01 ERA over 164.1 innings. He made the third All-Star Game of his career in front of the hometown Seattle crowd and continues to shove.

His last two starts have been especially dominant, with game scores of 72 and 80, respectively. That game score of 80 that he achieved in his 7.0 innings of one-hit ball against the Kansas City Royals was the second-highest this season. 

Limiting walks and generating whiffs is his forte, as his 86th percentile walk rate and 85th percentile on Baseball Savant prove his dominance in the categories. 

Castillo racks up strikeouts at a higher clip than Kirby, with 9.9 per nine innings in 2023, a total of 181 to this point in the season. However, his WHIP is his best stat, leading the entire league in the category at 1.004. 

The case for George Kirby 

If Castillo is dominant at limiting walks, Kirby is generational. He is in the 100th percentile in walk rate, as he lets just 0.8 free bases per nine innings. With 8.2 strikeouts per nine also being good, his 10.14 strikeouts per walk is the best in the league.

His control of the zone has led to a 10-8 record and a 3.28 ERA over 156.1 innings. Making 25 starts to Castillo’s 27, Kirby can usually push an out or two longer than his counterpart, but the gap is not that large. 

Kirby has had two games with higher game scores than Castillo’s season-best of 82, with a performance of 88 against the Baltimore Orioles Aug. 12 where he went the complete game, giving up just three hits. The other is his May. 31 performance against the New York Yankees, which was good for a game score of 83. 

He has even stepped up his game against the best opponents he faces, as he has posted 79.2 innings, a 1.81 ERA and a 0.86 WHIP when taking the mound against teams with a winning record. 

Verdict of the Mariners ace

Due to the nature of the two styles and mentalities, it is not easy to pick an answer. If the opponents have a high chase rate and swing early and often in counts, Castillo is the clear pick. But the nod should go to Kirby if they have a more balanced attack.

Having only two starts of his first 25, giving up multiple walks in a game, his uncanny ability to shut down good teams and his ability to push deeper in games, Kirby will usually give the Mariners the best chance to win in a win-or-go-home setting.