Mattheus was sent down on August 15 after posting a 14.09 ERA in his last 10 games with the Nats, but he allowed just one earned run in his five appearances (spanning 5 1/3 innings) at Syracuse, striking out three and walking none in that time.
Mattheus has a 6.26 ERA in 23 big league appearances this season.
The roster move brings back a guy with a good bit of major league experience, but it again leaves the Nats with just one left-hander in their bullpen. With Ian Krol having been optioned to Syracuse last week, Fernando Abad is the lone southpaw in the Nats' bullpen.
Cedeno again got a stint with the Nationals without actually appearing in a game. He's now been up with the Nats four times this season and has appeared in just two games, throwing 1 1/3 scoreless innings in that time.
The lefty came up to the Nats last Friday with Jhonatan Solano when the Nats cleared two roster spots by trading David DeJesus and Kurt Suzuki.