Mattheus, 31, made a single appearance at the big league level for the Halos after joining the organization on a minor league deal. While it is hard to learn much from one appearance, Mattheus worked at or above his usual two-seam fastball velocity (between 93 and 94 mph). He had a nice run to start the year at Triple-A, tossing 12 2/3 frames over which he permitted just four earned runs and registered 12 outs on strikes while not issuing a single unintentional walk.
The Reds have, of course, had rather pronounced bullpen struggles, and will hope that Mattheus can add some quality innings of middle relief. He proved a useful arm for several seasons with the Nationals, working to a 2.84 ERA over 98 1/3 innings from 2011-12. Though his results have declined since, and he missed some time with injuries, metrics suggest that he was much the same pitcher.
Mattheus, known most for his sinker and strong groundball induction rates, will of course require a 40-man roster spot, though the Reds appear to have an opening already. Cincinnati recently designated veteran righty Kevin Gregg after his difficult start to the year