Acquired from the Astros in April 2013, Cedeno, 27, appeared in 11 games and worked to a 1.50 ERA with Washington last season.
Treinen, 25, was used as a long man and made his Major League debut with the Nationals on April 12, pitching two scoreless innings against the Braves.
In three Major League appearances, Treinen allowed just one earned run in 6 2/3 innings [1.35 ERA]. He earned his first Major League strikeout on April 12 when he got Braves third baseman Chris Johnson swinging.
Treininen will rest for three days before he returns to the rotation for Syracuse.
"He is going to build his innings down there, certainly work on his secondary pitches. He has a pretty electric fastball," Manager Matt Williams said. "He pitched really well for us."
Williams also left a possibility that Aaron Barrett could return to the big leagues by Tuesday. Barrett didn't allow a run in 4 1/3 innings and struck out six batters. He was sent down because he couldn't give the Nationals a lot of innings. He is known as a closer and was dealing with a minor finger injury.
"We don't know yet, depending what happens in the next few days, but he is doing fine," Williams said about Barrett. "He is competing and doing all the things that he needs to do. Once Tuesday comes, we'll see what happens there. For right now, X [Cedeno] is here and we need him."
Bill Ladson is a reporter for MLB.com and writes an MLBlog, All Nats All the Time. He also can be found on Twitter @WashingNats. This story was not subject to the approval of Major League Baseball or its clubs.