We use ERA for starting pitchers because runs are the name of the game. For batters we can't assign runs so neatly, thus stats like OPS (or more sophisticated ones) serve us well as a proxy for runs. For relief pitchers, I see no reason not to rely on OPS too, at least for initial analysis. League OPS right now is .741. The Twins team OPS on defense is an unsightly .825 (Oakland is second worst at .771). Only three pitchers on the staff have amassed an OPS below .741: Tonkin (.720), Pressly (.679), Abad (.583), and Boshers (.419). As relievers, all reflect Small Sample Size, so any stat is prone to a sudden swing. But these four guys are not the problem, and Abad in particular (who I will admit I was lukewarm about bothering to sign to that minor league contract) has done his job. You can look at the culprits by searching for "OPS" here: http://www.baseball-reference.com/teams/MIN/2016-pitching.shtml