Through the first 20 games, the Rangers have relied heavily on Mike Adams, Joe Nathan, Alexi Ogando, and rookie lefty reliever Robbie Ross. The quartet has pitched 39.2 innings, distributed almost exactly evenly among the three, putting them each on pace for around 80 innings each. Although this heavy workload is less of a concern for Ogando, who pitched 169 innings as a starter last season, it is likely unsustainable for the other three. Joe Nathan pitched only 44 innings last season after coming back from Tommy John surgery; Mike Adams pitched 73 innings last season, but just 66 the season before that and 46 the year before that; although Robbie Ross pitched 161 innings as a starter and probably has less risk than the previous two, he is still a developing arm feeling his way through the majors, and may be shut down early.
Compounding the problem is the type of inning the four have been pitching, not just the quantity. Using the ‘Leverage Index’ statistic (where the median is 1, 1.5 is considered high leverage, and below 1 is considered low leverage), it is possible to quantify the importance of innings pitched to the final outcome of the game. Looking at the gmLI, or LI when the pitcher enters the game, Nathan leads the team at 2.09, followed by Mike Adams at 1.54, Ogando at 1.45, and Robbie Ross at 1.36. There is a significant gap to the next pitcher, Koji Uehara who checks in at a much, much lower .32. Obviously Mike Maddux and Ron Washington have decided to maximize the innings they get from their preferred relievers, and so far the Rangers bullpen has led the league in FIP at 2.59, Win Probability Added at 2.46, and are third in the league in Shutdowns and have by far the fewest Meltdowns, at 2 (the league median is 10). While relying on four elite relievers has worked out quite well for them, the Rangers may soon find they need to start giving either Mark Lowe or Koji Uehara some high-leverage innings, if only to give the rest of the bullpen a breather.
Deciding between Uehara and Mark Lowe isn’t as easy as it may seem, however. In their limited exposure this season, both have pitched well. They both are striking out approximately 30% of the batters they face, have just one walk and three earned runs between them, all three of which were home runs. In fact, the long ball has been their greatest weakness, as neither has benefited much from luck: both have BABIPs that are roughly near their career averages, and although Mark Lowe has left 100% of baserunners on base compared to a career average of 70%, the small sample size for that data prevents us from really drawing any conclusions.