On today’s list of the #26-#50 players in baseball according to the ESPN 500, Marlins ace Josh Johnson  came in at #30, one spot behind fellow sometimes-injured starter Chris Carpenter . It’s easy to see why Johnson (and Carpenter, for that matter) ranked so highly, as they are dominating starters when they take the mound. Johnson’s career ERA- is 70, meaning that he’s prevented runs at a rate 30 percent better than an average pitcher over that time. To put that in perspective, Roy Halladay ‘s career ERA- is 71. On a per-inning basis, Johnson stacks up favorably against any pitcher in the game.
However, while Johnson’s quality level is extremely high, his quantity of innings pitched is relatively low. He’s managed just 725 innings across parts of seven seasons, and has managed to make more than 14 starts in a season just three times in his career. Persistent health problems have been an issue that Johnson just hasn’t been able to shake, and given his lingering shoulder problems that limited him to just 60 innings pitched last year, the Marlins need to have muted expectations for how often he’ll be able to take the hill in 2012.
That realization is a large part of the reason that the team aggressively pursued Mark Buehrle  as a free agent. Buehrle might be the furthest thing from Johnson currently pitching in the game today – a soft-tossing left-hander who rarely gets a strikeout but who is a mortal lock to take the hill every fifth day, and pitch deep into the ballgame at that. Buehrle has reached the 200 inning mark in an astounding 11 consecutive seasons, and has never once been on the disabled list. In fact, he’s never missed a start for any reason since being called up to the big leagues in 2000. And yet, the ironman among current starting pitchers was rated only 134th by the voters in the ESPN 500, sliding in between Aramis Ramirez  and John Danks .
So, we know that the voters prefer quality over quantity in their starting pitchers, but now that the Marlins have both, which one is likely to provide more value for Miami in 2012?
These kinds of situations are the reason behind statistics like Wins Above Replacement, as baseball needed a way to measure the value of a player in relation to who might end up taking his place in case of an injury. Because WAR is a counting stat, rather than something like ERA, it allows us to compare the value of excellent performances in limited playing quantities against players who aren’t as valuable on a per-game basis but are more reliable and keep a team from having to turn to a replacement level player to fill a hole.
Considering Johnson’s many injuries since debuting in 2006 – Buehrle has thrown 540 more innnings during that time – you might expect that Buehrle would have a massive advantage in WAR while both have been big league starters, but it’s actually fairly close  – +20.8 for Buehrle, +18.5 for Johnson. Using a straight average, Buehrle has averaged +3.5 WAR per season over the last six years, while Johnson has come in at +3.1. They’ve gotten there very different ways, but the end results suggest that they’ve been similarly valuable to their respective franchises.
However, the ESPN 500 is about 2012 performance, and a straight average might underrepresent Johnson’s value since it includes entire years where he barely pitched while recovering from surgery. The potential value he represents if he stays healthy – as he did in 2009 and 2010, when he averaged +6.0 WAR per season and was the third best pitcher in the National League – far surpasses what the Marlins could reasonably expect from Buehrle going forward.
Buehrle can be reasonably expected to be worth about +3.5 WAR this season, as he’s been between +3.4 and +4.6 in every season since 2007 and is one of the most consistent pitchers in the game today. How many innings would Johnson have to throw to exceed that total? If you take Johnson’s career WAR of +18.6 and divide it by his 725 innings pitched, you get .025 WAR per inning. To exceed +3.5 WAR, Johnson would have to throw 138 innings this season. Interestingly enough, Dan Szymborski’s ZIPS projections have Johnson pegged for 129 innings this year, suggesting that Buehrle and Johnson may end up with very similar values for 2012.
While it’s understandable that the voters preferred upside over boring old durability, the respective histories of these two pitchers suggest that the Marlins best pitcher this season might not be Josh Johnson after all, and perhaps voters underestimated just how valuable the Marlins new workhorse can really be.