On Monday, the Reds designated veteran reliever Kevin Gregg for assignment after he posted a 10.12 ERA in 11 appearances. Cincinnati's bullpen entered the day ranked 14th out of 15 National League clubs in ERA.
"I think we have the talent here, the talent has to reach the level of our expectation and the expectation of each individual pitcher," Reds manager Bryan Price said. "The physical ability is in this bullpen to get the job done with who we have. Any team is going to need a number of pitchers to come in there and position players to do a job when we have injuries or a lack of performance. There could be some opportunity for Ryan to give us a shot in the 'pen."
The 31-year-old Mattheus spent 2011-14 with the Nationals and was managed in his first season by current Reds third-base coach Jim Riggleman. Mattheus' best season was 2012, when he worked a career-high 66 games and posted a 2.85 ERA with a 1.15 WHIP.
"He pitched effectively for us," said Riggleman, who resigned during the '11 season. "Then when I left, he just gradually got more and more into some prominent innings there and really did a nice job for Washington for a couple of years."
Struggles and injuries marred Mattheus' last two seasons. He had a 6.37 ERA and a 1.90 WHIP over 37 appearances in 2013, a season that included his going on the disabled list with a fractured right hand from punching a locker.
Last season, Mattheus totaled only 8 2/3 innings in the big leagues and spent much of the year at Triple-A Syracuse, where he posted a 5.80 ERA and 1.46 WHIP in 40 1/3 innings over 34 games. He had a rib cage injury during Spring Training and also dealt with a chest injury.
In 146 career relief appearances in the Majors, Mattheus is 7-7 with 3.58 ERA. The Angels signed him in January to a Minor League contract, and he was 0-2 with a 2.84 ERA in 11 appearances for Triple-A Salt Lake this season.
"I saw him for Syracuse when I was at Louisville," Riggleman said. "We spoke and talked about his progress coming back. Sometimes it takes that much time removed from an injury to get back to 100 percent, but it appears he is back."
To make room for the callup of Wednesday's starting pitcher, Raisel Iglesias, the Reds optioned right-handed reliever Carlos Contreras to Triple-A Louisville. Contreras was recalled on April 27 and had a 2.70 ERA in three appearances.
Mark Sheldon is a reporter for MLB.com. Read his blog, Mark My Word, follow him on Twitter @m_sheldon and Facebook and listen to his podcast. This story was not subject to the approval of Major League Baseball or its clubs.