The St. Louis Cardinals' pitching depth & the definition of 'replacement'

A year ago, the St. Louis Cardinals started the season with the following starting rotation:

1. Adam Wainwright
2. Jaime Garcia
3. Lance Lynn
4. Shelby Miller
5. Jake Westbrook

Injuries and ineffectiveness knocked Garcia and Westbrook out of the rotation after 55 1/3 and 116 2/3 innings respectively. But the unfortunate disabling of 40% of the Cardinals rotation was not entirely unexpected.

Earlier this year, Jeff Sullivan wrote a post at Fangraphs entitled, "Revisiting the Myth of the Five-Man Rotation." Sullivan delved into the 2013 numbers and found that very few MLB clubs actually used only a five-man rotation throughout the 162-game season. He summarizes his findings regarding the stats for pitchers who started in 2013 but weren't in the opening-day rotation of their clubs as follows:

Overall, these guys made 967 combined starts, for an average of 32 per team. They threw a combined 5,097 innings, for an average of 170 per team. They totaled 7.2 RA9-WAR, and 24.1 regular WAR, for averages of 0.2 and 0.8. Their combined ERA- was 125; their combined FIP- was 119.

As a whole, the Cardinals' replacement starters weren't far off the MLB averages that Sullivan shares regarding workload. The St. Louis replacements made 36 starts on the 2013 season and totaled 201 innings. So the Cards' 5.58 IP per replacement start is a bit higher than the MLB average of 5.27 IP per replacement start. The Cardinals' replacements threw more innings per start than the MLB replacement starters as a whole, but how did they perform during those innings?

Before going any further, let's revisit how replacement level is determined. Writing for Baseball Prospectus on WARP, Russell Carelton addresses what the term means, using Mike Trout and center field as an example:

Had Trout himself disappeared, the Angels probably would have responded by playing Peter Bourjos and Torii Hunter more often. But we don't want to credit or blame Trout for the presence of other players who just happen to be on his team, so we take an average of what everyone else's bench players might have done in Trout's place, rather than compare him just to the Angels’ backup options. Then we look at how much value those backup center fielders, on average, would have provided in the amount of time that Trout played last year.

Replacement level is a mathematical abstraction in that no such "replacement player" actually exists—you can’t point to Larry over there and say that he is the gold standard of replacement level. But really, a replacement player is just the per plate appearance (or per inning) mathematical (weighted) average performance of all backup center fielders, multiplied by the number of plate appearances (or innings) that Trout (or any other player whose value we want to assess) played.

In using this composite sketch of the state of backups in MLB, we trade the ability to answer the question, "What really would have happened to the Angels if Trout had vanished into thin air?" for the ability to compare everyone in MLB against a common baseline.

We can readily apply this same principles to starting-pitcher replacements. And the plateau of replacement level is something well below average production. Replacement-level players aren't very good; that's why they're readily available filler in every organization.

Now let's consider the Cards' replacement starters. The following chart shows each of them performed when starting in 2013:

The Cardinals' replacement starters put together a very good run last season. Michael Wacha led the charge, posting 0.9 fWAR. Joe Kelly (0.5), Tyler Lyons (0.3), and John Gast (0.1) all turned in performances above replacement level (however incrementally). And at -0.1 fWAR, Carlos Martinez was slightly below replacement level in his only start (however incrementally). Adding them together, the Cards received 1.7 fWAR from their replacement starters.

In 2013, National League starting pitchers posted a collective ERA of 3.86 and a FIP of 3.82. The Cardinals' replacement starters last season combined for a 3.45 ERA and 3.87 FIP. Last season, the Cardinals' pitching depth allowed the club to redefine "replacement level" for starting pitchers to be above average in run prevention and slightly below average in terms of strikeouts notched, walks issued, and homers allowed on a per inning basis. That's an impressive feat, and one of the reasons the Redbirds were able to rack up 97 wins. With Tyler Lyons making the club's first replacement start tonight against the Mets, we'll begin to see if the Cardinals' pitching depth will allow them to define up "replacement level" again in 2014.

Editor's Note: This post has been updated thanks to Craig, who pointed out that individual starter fWAR can be found on the Fangraphs leaderboards.