Quality of opposition is frequently cited in pitcher evaluation. There’s a pretty good reason for that. The more innings a pitcher throws, the more likely his quality of opposition will end up somewhere between five percent below and five percent above average. Things even out. The randomness of scheduling sometimes results in a pitcher landing in a bit higher or lower. This happened in 2014 to Jordan Lyles, who faced a tougher slate of opponents than usual. The metric used to identify strength of competition is Baseball Prospectus’s oppRPA+, which sets 100 as league average, with figures above or bellow 100 representing stronger or weaker competition.
In 2015, the Rockies had seven pitchers who threw more than 30 innings and faced offensive opponents at least seven percent better than league average. That was quite a lot. The Diamondbacks, Dodgers, Giants, and Padres combined to have seven pitchers who met the same criteria, so it wasn’t the National League West norm.
Kahnle and Axford faced the most difficult competition in 2015, at 12 and 11 percent above league average respectively. They’ll each be plying their trade for different teams in 2016, so we’ll just note that the tough competition might have contributed to their up and down seasons. The next four names are worth taking note of because each one is likely to play a big role in the Rockies 2016 bullpen. In 182 ⅓ collective innings pitched in 2016, Oberg, Logan, Friedrich, and Miller faced an opposition that was, as a whole, about eight or nine percent better than league average.
As Nick Stephens noted earlier, the Rockies’ bullpen outlook for 2016 is promising. Oberg and Miller have shown flashes that they can be effective late inning relievers, Friedrich has held his own as a full-time lefty reliever, and Logan can be recast as a left-handed specialist. One way to view their strength of opposition in 2015 is to expect less exposure to stronger lineups in 2016—a sort of oppositional regression to the mean. Seen that way, the bullpen’s outlook is even better for these important relief pitchers.
From another perspective, the strength of opposition numbers from 2015 might predict rebounds for individuals, but not for the team. The division is still offensively strong. According to Baseball Prospectus, the NL West had three of the five best offenses in the National League (the Dodgers, Giants, and Diamondbacks). This accounts for the tough slate these pitchers saw in 2015. The limited innings made it more likely for relievers to finish the year having faced exceptionally tough opponents. It also means 2016 might be just as tough. If Oberg has an easier year, for instance, it might just mean that the difficulty will have been displaced to other members of the staff.
The same caveats do not apply to the final name on the list: Jon Gray. In Gray’s 40 ⅔ innings pitched in 2015, he faced opponents who were about seven percent better than league average. While his ERA doesn’t reflect it, he did pretty well in those innings. As opposed to the relievers, Gray will likely pitch enough innings for the level of competition to trend much closer to league average. None of the four Rockies pitchers who threw over 100 innings in 2015 had a strength of opposition higher than three percent above league average. If Gray couples continued development with a relatively easier slate of opponents, 2016 might be a big year for him.
Strength of opposition is something that is only notable when the results land at the extremes. In 2015, the Rockies had six relievers who faced an exceptionally strong slate of opponents. That competition is going to remain, although that doesn’t mean 2016 will just repeat 2015. Baseball is unpredictable, after all. Noting the strength of opposition is more important for Gray. In what will be his rookie season, a slightly weaker set of opponents for the season has the chance to result in Gray's emergence as a top of the rotation starter.