This is a topic I've brought up on Sports Open Line and on the Countdown to Opening Day in recent weeks. I figured I could get into a little more depth here than I can on the air ... so here goes nothing. Reminder: everything that follows is based on reasonably good health, as are all pitching projections.
For the purposes of this discussion I'm going to leave out Adam Wainwright (free agent) and three others who are going to be members of the rotation, barring injury, in Jack Flaherty, Miles Mikolas and Kwang Hyun Kim. The other internal rotation candidates will be discussed here simply because there's a chance any of them could wind up in the bullpen rather than the rotation.
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Left handers
Genesis Cabrera
Austin Gomber
Andrew Miller
Tyler Webb
Miller and Webb are relievers, plain and simple, while Cabrera and Gomber could also be candidates for the rotation.
Miller isn't the dominant force he was prior to arriving in St. Louis but his slider is still an elite weapon (opponents hitting .187 on his slider since he joined the Cardinals) and he's been awfully tough on lefties. Webb has been very solid (3.03 ERA in 104 games 2018-2020) and while he's not a hard thrower or big strikeout guy he has been effective at limiting hard contact, ranking in the top 10% in MLB in opponent exit velocity in 2020 and in the top 5% in that category in 2019.
Cabrera could get a chance to start in Spring Training - 92 of his 114 minor league appearances came as a starter - and he has the pitches/stuff to do it. The problem for Cabrera is command, especially as a starter, so the bullpen might end up being the best option for him. He could be a dominant multi-inning reliever and/or late inning leverage reliever based on his stuff. With a fastball that has averaged more than 96 MPH (top 12% in MLB in 2020) in the big leagues, Cabrera generates a ton of swing and miss. In 2020 his strikeout rate ranked in the top 9% in MLB and his whiff rate (swings and misses) ranked in the top 2%. 2020 was a small sample size but it's easy to see that he has elite stuff and is a real weapon heading into 2021.
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Gomber isn't the hardest thrower (average fastball 92.5 MPH in 2019-2020) but he has a nice 4-pitch mix and a long history of being an effective starter in the minor leagues. He'd probably add a little velocity if he were used solely as a reliever but, again, velocity is not his game. In 2020 he was more effective as a starter, though he was fine in the bullpen too, but it was a small sample size so it's hard to put too much weight on that. Still, I like Gomber a lot and I think his fastball-curve-slider-change combo is plenty good enough to make him a mid-rotation starter or a quality reliever.
This is a really strong group of lefties to start with and that's not counting prospects who aren't yet on the 40-man roster like Zack Thompson and Matthew Liberatore.
Right handers
Giovanny Gallegos
John Gant
Ryan Helsley
Jordan Hicks
Carlos Martinez
Alex Reyes
Johan Oviedo
Daniel Ponce de Leon
Kodi Whitley
Gant, Helsley, Martinez, Oviedo, Reyes and Ponce de Leon could all be rotation candidates, depending on how things go when they stretch in out Spring Training, but Martinez, Oviedo and Ponce de Leon seem more likely to pitch at the back end of the rotation than the others. Gallegos, Hicks and Whitley are straight up relievers.
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Martinez has been on a wild, bumpy road these last three seasons (and he was terrible in 2020) so this is simply a question of where his stuff plays best and how healthy he can stay. In 2019 when he was used exclusively as a reliever, his fastball velocity wass in the top 15% of MLB (96.6 MPH on his 4-seasm FB) and his strikeout rate was good. Last year his 4-seam FB dropped to 93.3 MPH but he also had a rough bout with COVID and missed time. He has the pitches to start but does he have the physical ability to log heavy innings at this point or would he be better served as a multi-inning or late-inning reliever going all out all the time?
Ponce de Leon has great stuff and can get a lot of swings and misses but his command needs to improve (bottom 10% in walk rate in MLB 2019-2020) for him to be a consistent starter at the Major League level. If he throws strikes he's tough and his curve ball is a real weapon. Could be helpful as either a starter or a reliever.
Gant is interesting. He's been a quality late-inning reliever for the Cardinals the past couple of years but he has the history and the repertoire that show he could be a quality starter. 107 of his 113 minor league appearances came as a starter and back in 2018 he posted a 3.61 ERA in 19 starts for the Cardinals. His fastball plays up a bit in the pen compared to what it would likely be when pitching 5-6 innings but his change is such a weapon that the small velocity drop wouldn't matter. We know he is a quality reliever, it will be interesting to see what kind of change he gets to start this spring...depending on who is healthy and on the roster at that time.
Helsley is interesting as a starter as well, though with his high spin rate fastball that touches 100 MPH he might be best used either as a multi-inning or late-inning reliever. 69 of his 82 appearances in the minors came as a starter but since he's never started a Major League game we don't know how that would look. Despite his high octane fastball (97-98 MPH on average in the Majors) he hasn't been much of a strikeout pitcher, which is interesting.
The dream of Reyes as a starter still lives - he has the stuff and the repertoire to do the job - and it will be fascinating to see how things go this spring. It might be best for his health to limit his workload rather than asking him to be a 150-180 inning guy in the rotation moving forward but the temptation to see how he looks in the rotation has to be strong. Even though 2020 provided a small sample size it is interesting to note that Reyes ranked in the top 11-15% in whiff rate and strikeout rate while ranking in the top 3% in fastball velocity (97.5 MPH average 4-seam FB). Reyes' curve ball spin rate was in the top 11% and his fastball spin rate was in the top 6%, hence all the swings and misses. Reyes is dominant, without doubt, it's just a matter of figuring out how to best keep him healthy.
If Oviedo doesn't make the Cardinals' rotation in spring training I'd expect him to be back at Memphis working as a starter rather than in the Major League bullpen. His future sure looks like it's going to come as a big league starter. He's a big man with a big fastball that would surely play up in the bullpen but he seems to have the ability to be a big innings guy, hence my belief that he's better off starting than relieving. For now.
We haven't seen much of Whitley at the Major League level but since only 2 of his 104 minor league appearances came as a starter it's safe to assume he'll be a reliever if he's in the Majors. He has mid-90's fastball, a slider and a change at his disposal. The Cardinals really like him and he seems likely to have a bullpen role in the Majors this year if he performs well in Spring Training.
Gallegos has been one of the most underrated relievers in baseball the past couple of years. With a fastball that sits around 94 MPH he's not the blow-you-away-with velocity reliever that some others are but he gets a ton of swings and misses. He has struck out 11.5 batters per 9 innings in 2019-2020 and that's because of a slider that opponents have hit .095 against during that same time frame. He can work as a closer or as a high-leverage guy earlier in the game and he's equally good against right and left handed batters.
Hicks will be nearly 20 months removed from Tommy John surgery (recovery is typically 12-15 months) when Spring Training starts so we should see a guy who is completely healthy, at least in terms of the elbow, when things ramp up. He was the hardest thrower in baseball in 2018 and 2019 so even if he loses a bit, which isn't a certainty, he'll still be near the top of the league. Hicks also has a slider that opponents have batted .118 against in the Majors so when he's back he's a major late-inning weapon, probably the closer with all of the other elite arms being more likely to handle multiple inning usage in 2021.
So, that leaves the Cards with three right handers blowing 100+ MPH and a handful of others who have multiple high quality Major League pitches.
Keep in mind, that's a group of 13 pitches listed above without even counting Seth Elledge, Junior Fernandez, Angel Rondon or Jake Woodford, all of whom are on the 40-man roster. It also doesn't count a possible Wainwright return.
With reasonably good health this should be one of the 3-4 best bullpens in Major League Baseball in 2021.