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5/19/2013 1:11 A.M. ET

Harper gets day off with sore left knee

SAN DIEGO -- Nationals outfielder Bryce Harper was out of Saturday's lineup against the Padres because of soreness in his left knee. Harper hurt the knee after he banged into the wall Monday against the Dodgers.

Besides the knee injury, Harper jammed his left shoulder, suffered a sore abdomen and nausea. He had an MRI on Friday to check on the abdomen and the results came back negative.

"Everything else feels pretty good," Harper said. "The knee was barking. It's pretty swollen. I'm not going to take a chance on it. I'll get some treatment on it and see if I can go tomorrow."

There have been a lot of arm-chair observers who insist that Harper suffered a concussion on the play Monday, but Harper said the Nationals did everything to make sure that he was OK. Harper saw two doctors in Los Angeles, three days in a row.

"I feel better. My head feels fine," Harper said.

Duke to get the spot start on Monday vs. Giants

SAN DIEGO -- Nationals manager Davey Johnson has decided that left-hander Zach Duke will start Monday night against the Giants at AT&T Park.

Johnson picked Duke over right-hander Craig Stammen because Johnson didn't want to break Stammen's routine as a reliever. Stammen is having another great season in the bullpen. In 11 games, he has a 2.25 ERA.

Johnson hasn't been able to find enough work for Duke this season and the skipper believes a start will get him going. As a long reliever, Duke has allowed 14 runs in 15 innings.

"Stammen has been very good. I don't want to change him out of the role that he is in," Johnson said. "With Duke, he has been basically a starter. I haven't able to give him the kind of work to keep him sharp. This will help him get back to where he needs to be. It kills two birds with one stone."

This will be Duke's first Major League start since 2011 when he was with the D-Backs. Duke has spent most of his time with the Nationals as a reliever. He is replacing left-hander Ross Detwiler in the rotation because Detwiler injured his lower back against the Dodgers on Wednesday. Detwiler had a third MRI on Saturday and, it turned out, he has a right oblique strain. It's not known if he will go on the 15-day disabled list.

Duke said he is appreciative that Johnson is giving him a chance to start. Pitching coach Steve McCatty informed him of the assignment Saturday afternoon, and the left-hander immediately had a bullpen session. In four games, Duke is 1-0 with a 1.08 ERA at AT&T Park.

"I'm sorry about what happened to Ross. I hope it's nothing very serious. I hope he can come back quickly," Duke said "At the same time, I have to get out there and give us a chance to win the game and re-prove to the organization and to the guys in this clubhouse that I can pitch and get people out."

If Detwiler is out for more than one start, the Nationals will call up a pitcher up from the Minor Leagues. There is a chance it could be a reliever, while they add Duke to the rotation.

Worth noting

• Manager Davey Johnson announced that outfielder Jayson Werth will miss the three-game series against the Giants because of a right hamstring injury. He was scheduled to rejoin the Nationals on Saturday against the Padres, but still has tightness in his hamstring. Johnson said that Werth went to Chicago to see a hamstring specialist and will see the team doctors in DC on Monday.

• Johnson believes catcher Wilson Ramos' hamstring strain is worse than the last hamstring injury he had. Ramos will be out longer than 15 days, according to Johnson.

Bill Ladson is a reporter for MLB.com and writes an MLBlog, All Nats All the time. He also could be found on Twitter @WashingNats. This story was not subject to the approval of Major League Baseball or its clubs.