There are a few pitchers who have been dominant and were once starters in the Cardinals' system; however, they did not see major league innings. Therefore, they do not qualify for this exercise. Zac Gallen would be the most notable pitcher who doesn't qualify, as he never pitched a major league inning for St. Louis.
No. 1 Starter: Jordan Montgomery
While Jordan Montgomery isn't the typical dominant "ace" of a team's starting rotation, he has been a very steady starter for many years now. Sadly, the team's hand was forced this year, and Montgomery was traded at the deadline to the Texas Rangers.
Monty has a 3.47 ERA, 1.221 WHIP, and a 3.55 FIP across 168.2 innings this year. He strikes out about 7.8 batters per nine innings, but he does a great job at limiting home runs and walks. Montgomery would be a very fine "ace" of the Cardinals staff in 2023. Assuming he signs elsewhere this offseason, Montgomery's steady pitching output will be missed in St. Louis.
No. 2 Starter: Sandy Alcantara
In 2022, Sandy Alcantara put on a pitching showcase that hadn't been seen in a long time. He threw well over 200 innings, struck out 207 batters, and won the Cy Young Award. Cardinals fans were rightly furious at the front office for trading away a stud like Sandy.
2023 has been a bit different for the tall righty; while his numbers are still very satisfactory, they aren't quite what Marlins fans became accustomed to in 2021 and 2022. This year, Alcantara has a 4.14 ERA, 4.03 FIP, and a 1.213 WHIP. He isn't striking out as many batters as last year, and he is more prone to homers, but he is still a dominant pitcher who will go deep into games regardless of the score. Alcantara would be a very strong #2 pitcher for the Cardinals this year.
No. 3 Starter: Michael Wacha
The St. Louis Cardinals had one of the best rotations in 2013. The starters that year were either established studs (Wainwright), or up-and-coming youngsters. Michael Wacha was in the latter group. Wacha played 7 seasons in St. Louis and pitched to a 3.91 ERA, 3.96 FIP, and an ERA+ of 102.
The Cardinals chose to not resign Wacha in 2019, and he then spent the next four years playing for a variety of teams. The San Diego Padres signed Wacha this past offseason, and he has not disappointed. This year, Wacha has a 3.43 ERA, 4.16 FIP, 1.214 WHIP, and an ERA+ of 119. Wacha is also striking out nearly 9 batters per nine innings. He would slot in very comfortably as the ex-Cardinals' #3 starter.
No. 4 Starter: Jose Quintana
At the Trade Deadline in 2022, the St. Louis Cardinals sent Johan Oviedo to the Pittsburgh Pirates in exchange for the veteran Jose Quintana. Quintana pitched well enough for the Cardinals in the second half of 2022 that he was chosen to start the first Wild Card game that year in the playoffs. The Cardinals chose to not re-sign him, letting him go in free agency.
Quintana started the year on the Injured List for the Mets, but since his return, he has pitched admirably. Through 66 innings this year, Quintana has a 3.02 ERA, 3.38 FIP, 1.279 WHIP, and an ERA+ of 139. Quintana would be a very strong fourth starter in a rotation for any team, especially a playoff-hopeful one.
No. 5 Starter: Jack Flaherty
Jack Flaherty was another pitcher whom the Cardinals traded at this year's deadline. While Flaherty was once a top-5 Cy Young candidate, he was unable to reach those heights again in St. Louis. Injuries and ineffectiveness hampered his ability to control the game.
Flaherty was sent over to the Orioles, where he has struggled. However, on the year, Flaherty would be a very serviceable fifth starter. Across Baltimore and St. Louis in 2023, Jack has a 5.03 ERA, 4.40 FIP, 1.578 WHIP, and an 85 ERA+ across 144 innings. While these numbers aren't ideal for a young, talented pitcher, they would be just fine for a fifth starter.