Cabrera is 0-5 with a 5.95 ERA and leads the National League in walks (32) and wild pitches (nine).
Washington wants Cabrera to work on his delivery with pitching coach Randy St. Claire. When he has a quick delivery, Cabrera is able to throw strikes. When he slows it down, he has a propensity to walk too many batters.
Cabrera also has a tendency to put his right cleat on the rubber when he pitches, but most pitchers have their foot in front of the rubber.
"We spoke to Daniel yesterday and we hope to relax him [in the bullpen] so he can continue to work [with Randy] and earn his way back in the rotation," Nationals manager Manny Acta said.
Cabrera, who took the news like a professional, acknowledged he has never been a reliever and the adjustment could be tough.
"It doesn't feel comfortable to me. Of course, I've never been there, but I'm an employee of this team. Whatever they ask me to do, I have to do it," Cabrera said. "I have to throw strikes. That has been the biggest concern of my career. It has been worse than ever. I just have to keep on working. It has been terrible. Nothing good happens when I go out there -- something bad happens."
With Cabrera out of the rotation, right-hander Craig Stammen will start against the Pirates and make his Major League debut on Thursday. Stammen, one of the team's top pitching prospects, was 4-2 with a 1.80 ERA with Triple-A Syracuse. One reason Stammen, 25, has been impressive is the effectiveness of a two-seam fastball he developed during Spring Training.
"Stammen fits the rotation requirements. His turn was yesterday and he has earned it," acting Nationals general manager Mike Rizzo said. "He has pitched outstanding in Triple-A. The two-seam fastball separated him from his performance last year."
Stammen opened eyes in the organization last season while pitching at three Minor League levels. He had a 1.64 ERA in six games for Double-A Harrisburg.
"He's a little bit of a late bloomer," Rizzo said. "He was a different pitcher last year. He was a power pitcher with a four-seam fastball that was pretty straight."
With Stammen included, the Nationals' rotation is filled with second-year and rookie pitchers. John Lannan is in his second season, while Ross Detwiler, Shairon Martis, Jordan Zimmermann and Stammen are rookies.
"It says we have a core of Major League starters and it says that the future looks bright," Rizzo said. "We hope the rookies come of age sooner than expected and that they will win a lot of games for us."
In order to put Stammen on the roster, the Nationals optioned right-hander Garrett Mock to Syracuse after Wednesday's game against the Pirates.
Like most of the relievers in the Nationals' bullpen, Mock had problems throwing strikes, allowing eight walks in 13 innings this season. He also gave up 10 runs in those 13 frames.
"I have some packing to do," Mock said. "I have to throw strikes. I didn't throw enough of them, I guess."
Bill Ladson is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.