The 26-year-old Storen was sent down on July 27 after going 3-2 with two saves and 5.95 ERA in 47 games. He had no record and a 5.68 ERA in six games with the Chiefs, but tossed 2 1-3 scoreless innings over his last three appearances in the minors.
Storen had 43 saves in 2011, and then missed the first part of last season due to right elbow surgery. He came back in July and finished with four saves and a 2.37 ERA over 30 1-3 innings, but he blew a two-run lead in Game 5 of the NL division series against St. Louis, ending Washington's season.
Washington signed veteran closer Rafael Soriano during the offseason, moving Storen into a setup role.
"The way I look at it, this whole year in general, this is kind of like opening day for me," Storen said. "I can't really control or change what's happened this year, but one thing I can do is really be effective here at the end and help these guys win."
With Syracuse, Storen worked on changing his delivery to a more conventional leg kick and away from keeping the leg straight.
"It was good to get back to throwing athletically and not have to think about it so much," Storen said. "I was getting to the point where I was thinking a little too much about what I needed to do mechanically instead of just lifting and throwing, and throwing strikes and attacking hitters."
Mattheus, 29, had a 9.45 ERA in nine appearances since returning from a broken right hand that occurred when he hit a locker. He is 0-1 with a 6.26 ERA in 23 games overall.
Manager Davey Johnson said Storen will move back into the setup role he shared with left-hander Tyler Clippard before he was sent down.
"For us to win a bunch of games I need that late-inning stopper and Mattheus, since coming back, his command of his pitches hasn't been as sharp as they were prior to him slamming that locker," Nationals manager Davey Johnson said.