Merrill Kelly
Merrill Kelly silently was an ace-caliber starting pitcher last year. You could make the argument that he pitched better than Gallen. Regardless, Kelly's 2023 season solidified him as one of, if not the most underrated starting pitcher in baseball. It's a crime he didn't receive one Cy Young vote last season.
Through 177.2 innings, Kelly owned a 3.29 ERA, 3.85 FIP, and 1.19 WHIP. While his 9.6% walk rate wasn't all that impressive, he also turned in a 25.9% strikeout rate and a 1.01 HR/9. While Kelly wasn't as bad as Gallen at preventing hard contact, his 89.6 MPH exit velocity and 41.8% hard-hit rate were both below average. Although his 7.8% barrel rate clocked in the 51st percentile among pitchers.
Kelly made up for Gallen's Postseason struggles during October. In 24 innings, Kelly only allowed six earned runs, struck out 28, and only walked eight batters. During his lone World Series start, Kelly fired off seven strong innings, allowing just one run, but struck out nine and, impressively, did not allow a single free pass.
Merrill Kelly has been a great story, going from a career minor leaguer, re-inventing himself in the KBO, and now one of the best pitchers in baseball with Postseason success. Kelly may end up being the best starter in the Diamondbacks' rotation next year. That's a pretty bold statement, but I wouldn't put it past Kelly.