Somebody had to pitch at the front end of the 2015 Reds bullpen, and that person turned out to be Ryan Mattheus in large part. He didn't make much of a case for a promotion to working the later innings, but he didn't single-handedly lose a bunch of games or have terrible results, either. The Reds project to have a bunch of inexperienced starters in the rotation again next year, and in such cases you are going to need guys in the bullpen who can chip in a lot of not-terrible low-leverage innings and keep the team in the game. That's pretty much what Ryan Mattheus is.
By the Numbers
57 G, 55.0 IP, 4.09 ERA, 97 ERA+, 1.53 WHIP, 5.7 K/9, 2.8 BB/9, 53% GB%
Mattheus isn't a strikeout artist, and far from it. But he does get a lot of groundballs. That's the one key piece of the puzzle that allows him to be as effective as he is, because it allows him to keep the home runs in check. Mattheus is extremely hittable, to the tune of 11.0 H/9, which was matched in badness only by Kevin Gregg among pure relievers last year. As a consequence, the WHIP was better than only Gregg and Colin Ballester, and those guys hardly pitched any innings. The low K-rate and one of the worst soft-contact percentages in MLB are indications that Mattheus just doesn't often fool hitters. That, and hitters hit nearly .300/.350/.400 against him.
But last year Mattheus did eat up a good share of innings, and that was a much-needed trait in itself for the Reds. A team (and especially one like the Reds) should probably expect the innings from the 5th arm in the bullpen to be somewhat below average. As long as he is shielded from facing left-handed hitters, Mattheus can be an effective reliever.
The bullpen is far from solidified for 2016. If Aroldis Chapman gets traded, the competition simply widens further. Mattheus is a known quantity and will be inexpensive, so he probably has an inside track at a spot in the front of the bullpen. However, there are a lot of arms vying for a limited spots, so Mattheus will have to perform well to keep his. Also, if trades happen to cause a downgrade for the Reds infield defense, Mattheus would probably feel the negative impact most keenly as an extreme pitch-to-contact guy, and if last season's unearned runs become earned this time around, he'll struggle to hold his job. Not unlike the 5th arm in most teams' bullpens.
Chance of making 2016 Reds roster: 75%