Given that both are playing well and nothing seems amiss statistically, it is best to look at their body of work over the course of their careers. In this respect, Koji truly shines compared to Lowe. He has had two solid seasons of relief experience where he was at or near the league lead in run prevention, and he did it all while on the Orioles squad, whose home park is hitter-friendly, although not quite to the degree of Rangers Ballpark. Mark Lowe, on the other hand, had one great season in 2009 where he threw 80 innings and limited opponents to a 3.26 ERA in spacious Safeco Field while with the Mariners; other than that, he has not been very effective. In 2011, he struggled slightly with home runs but also with walks, producing a 4.16 FIP over just 45 innings. Between Koji’s better track record and experience with a hitter-friendly park probably makes him the preferred choice to take on high leverage innings.
While Uehara is probably good enough to replace one of the top four in a limited capacity, the Rangers might be wise to explore the possibility of a transaction to replace a reliever who falls victim to a long-term injury. In that case, the Rangers will be limited to a trade, since there are no effective free agent relievers apart from Mike Gonzalez, and the Rangers appear to have been reluctant to re-sign him. Some possibilities include Brandon League from the Mariners, Grant Balfour from the Athletics, Brett Myers from the Astros, and possibly Joel Hanrahan of the Pirates. Huston Street in particular looks like a very attractive option; he is having a terrific season so far, leading the aforementioned group with a miniscule .82 FIP. However, any such trades will have to wait until these teams are out of contention. Moreover, the Rangers might face some serious competition for the limited supply of available relief help. For example, the Angels will probably be looking for an upgrade as soon as possible: their bullpen has the fewest Shutdowns in the league with just 5, while their bullpen has been responsible for 12 Meltdowns, tied for 5th in the league.
Until either Adams, Nathan, Ogando or Ross goes down, the Rangers won’t need to worry about replacements. And if Uehara earns Ron Washington’s trust and continues to perform well, the Rangers may never need to worry about replacements. Still, at this stage of the season, it is a concern that the four most important pitchers to the Rangers are on pace for 80 innings apiece.