Which pitcher has been the most indispensable this season?

In the aftermath of his worst start since coming stateside, Masahiro Tanaka hit the disabled list on Wednesday. The preliminary diagnosis is elbow inflammation, with the caveat that the Yankees' team doctor has yet to examine his MRI. (UPDATE: The MRI revealed a partially torn UCL that will sideline Tanaka for at least the next six weeks and possibly the rest of the season if Tommy John surgery is required.) For a team that's barely above .500 but still fancies itself a contender, it's a crippling blow, particularly given the weakness of the remaining rotation. All of which raises a question: Is Tanaka the game’s most indispensable pitcher?

While there exists no shortage of ways one could tackle this, we've chosen to take an objective approach using Wins Above Replacement (Baseball-Reference.com version), measuring the size of the gap between each team's top starting pitchers or position players and the next best on the roster. Admittedly, this is an imperfect methodology, particularly on the latter side, because half a season of a fielding metric can be quite noisy, and the sizes of the gaps are closely bunched together beyond a certain point such that the distinctions can look like hair-splitting. On the pitching side, we’ve chosen to rule out relievers, since it’s not as though one can truly pick up the slack for a missing starter.

What follows here are the six most indispensable pitchers — since there’s a tie for fifth, not to mention first and third — with the position players to come in a separate piece; I’ve broken the ties based on individual WAR.

1A. Adam Wainwright, Cardinals

Gap: 3.2 (4.7 WAR to Michael Wacha 1.5)

Wainwright gets the nod for the head of the pack, because while the gap on the Cardinals' side is equal to that on the Yankees' side, his 4.7 WAR trump's Tanaka's 4.2, and that's without including the extra half-win he's added with his bat via a .231/.268/.308 showing. Wainwright's 4.7 pitching WAR and 1.79 ERA both lead the NL, and while Clayton Kershaw (3.7 WAR and 1.79 ERA, albeit with too few innings to qualify in the latter category) is getting the attention thanks to his no-hitter and his 36-inning scoreless streak, the Cardinals' ace has stretched his stifling performance over 131 innings, 44 1/3 more than Kershaw.

With the sudden decline of Shelby Miller and the loss of Wacha to the disabled list due to a stress reaction in his shoulder, Wainwright has had to carry a larger share of the load than usual, which helps to explain why the Cardinals are merely second in the NL Central instead of first.

1B. Masahiro Tanaka, Yankees

Gap: 3.2 (4.1 WAR to Hiroki Kuroda 0.9)

Wainwright at least has other active pitchers in his rotation who are preventing runs at a better-than-average clip. The same can't be said for Tanaka, who has seen the Yankees' rotation collapse around him, with Ivan Nova and CC Sabathia both lost to season-ending injuries. The 25-year-old righty’s splitter-driven 13.7 percent swinging strike rate leads the majors; meanwhile, his 4.1 WAR is second in the league, and his 2.51 ERA and 7.1 strikeout-to-walk ratio are third.

Tanaka's 159 ERA+ is 60 points higher than the Yankees' second-best starter, David Phelps, and 66 points better than that of Kuroda, who has more innings and who finally appears to be showing his age. Both Kuroda's 4.20 ERA and 3.94 FIP are career-worst marks, the latter driven by his worst strikeout rate since his 2008 stateside arrival. And with Tanaka now out for an extended period of time, the Yankees can likely kiss their playoff hopes goodbye.

3A. Chris Sale, White Sox

Gap: 2.1 (4.2 WAR to Jose Quintana 2.1)

With Wednesday's strong start against the Red Sox, Sale moved into the AL leads in both WAR and ERA (2.08), having finally accumulated enough innings (95) to qualify for the latter. Though he missed roughly a month of the season due to a flexor strain, the 25-year-old southpaw has been more dominant than ever thanks to career lows in home run and walk rate (0.6 and 1.5 per nine respectively) en route to career bests in strikeout-to-walk ratio (6.4) and FIP (2.45). Quintana hasn't pitched badly at all (3.20 ERA, 2.88 FIP), but he's the only other White Sox starter with an ERA+ better than 100 (126 to Sale's 208).

3B. Yu Darvish, Rangers

Gap: 2.1 (2.9 WAR to Martin Perez 0.7)

By the numbers, Darvish has been better than ever; his 2.78 FIP is about half a run lower than his first two seasons, fueled by his best home run and walk rates (0.7 and 3.0 per nine, respectively) to date, not to mention a league-best 11.1 strikeouts per nine. As with Tanaka and the Yankees, the Rangers' rotation has collapsed around him; coupled with the injuries on the position player side, the team has lost 18 of 21 to fall to last place in the AL West. Perez showed brief signs of brilliance before scuffling and then being lost to Tommy John surgery in mid-May, but in a rotation that also lost Matt Harrison for the year and has yet to see Derek Holland, he's hardly unique. Nick Tepesch (4.29 ERA, 94 ERA+) has been slightly better than Perez in terms of overall run prevention, but his 4.98 FIP suggests disaster ahead.

5A. Felix Hernandez, Mariners

Gap: 1.9 (4.0 WAR to Chris Young 2.1)

With the league's second-best ERA (2.11) and best FIP (1.94), King Felix is in the midst of the best season of his career and — given the Mariners' resurgence — his most relevant one since 2007, the last time Seattle was a strong contender for a playoff spot. He's currently setting career bests in all three major peripheral categories, with 0.3 homers, 1.5 walks and 9.6 strikeouts per nine. Meanwhile, the oft-injured Young — who didn't pitch in the majors at all last year and made just 60 appearances from 2008-2012, has posted a 3.08 ERA in 105 1/3 innings. Given his 4.92 FIP, he's likely to be overtaken by HisashiIwakuma, who missed the season's first month due to a finger tendon strain but has been outstanding since returning.

5B. Mark Buehrle, Blue Jays

Gap: 1.9 (4.0 WAR to Drew Hutchison 2.1)

At age 35, Buehrle has suddenly started preventing runs at a career-best clip; his 2.60 ERA is more than half a run per nine better than his previous best, set in the White Sox championship season of 2005. His 3.74 FIP and typically unassuming peripherals tell a different story, which may explain why he's been touched up for a 4.09 ERA and 1.6 HR/9 in his last five starts, which have coincided with his team’s month-long slump. Meanwhile, the 23-year-old Hutchison has emerged as a solid pitcher (3.86 ERA, 3.73 FIP), albeit one who's kept on a short enough leash that he's averaging just 5.8 innings per turn.

Honorable mention:Henderson Alvarez, Marlins (Gap: 1.8), Corey Kluber, Indians (Gap 1.7), Julio Teheran, Braves (Gap: 1.4), Dallas Keuchel, Astros (Gap: 1.4).