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9/4/2013 7:41 P.M. ET

Davey sending Roark to start against Marlins

PHILADELPHIA -- The Nationals have decided to start right-hander Tanner Roark on Saturday against the Marlins instead of right-hander Ross Ohlendorf, who will go back to the bullpen as a long man.

Roark has been valuable in long relief since joining the team from Triple-A Syracuse on Aug. 6, but manager Davey Johnson said he deserves a chance to be in the rotation, based on scouting reports from Minor League pitching coordinator Spin Williams.

Johnson added that Ohlendorf going back to the bullpen is not a demotion.

"This is giving Tanner Roark an opportunity to start up here in the big leagues and see how he fits," Johnson said. "He was good out of the 'pen, but he is a young starter. Spin Williams really likes him as a big league starter. So we figured we take a look at that."

Ohlendorf went 2-0 with a 3.62 ERA in five starts, but what concerned the Nationals was that his pitches would slow by at least 10 miles an hour by the time he reached the sixth inning -- at one point Johnson thought he was running out of gas. Ohlendorf last pitched on Sunday, against the Mets, going five innings and allowing four runs in a 6-5 victory.

Johnson could have had Stephen Strasburg pitch on Saturday, but hamstring issues have been bothering Strasburg for a couple of starts, though he is expected to pitch on Sunday on six days' rest.

"It's not worth the risk-reward," Johnson said of using Strasburg early.

Harper gets a rest with sore hip still an issue

PHILADELPHIA -- Outfielder Bryce Harper found himself on the bench at the beginning of Wednesday night's game against the Phillies after getting treatment on his injured left hip earlier in the day.

Manager Davey Johnson asked Harper before the game how the hip is coming along, and Harper acknowledged that it is still hurting, so Corey Brown got the start in left field.

Because of the off-day on Thursday, Johnson believes Harper will be back in the lineup on Friday against the Marlins. Harper doesn't have any plans to get an MRI, and the origin of the injury is unknown.

It has been an injury-plagued season for Harper, who missed more than a month because of an injury to his left knee after banging into the wall at Dodger Stadium on May 13.

Mattheus acknowledges location has been a problem

PHILADELPHIA -- Right-hander Ryan Mattheus has struggled since returning from Triple-A Syracuse last week, allowing five runs in 3 2/3 innings.

Mattheus said that location -- keeping his sinker down in the strike zone -- has been his biggest problem.

In the sixth inning of Tuesday's game, against Phillies second baseman Chase Utley, Mattheus threw a pitch right down the middle, and Utley connected for an RBI single.

"Everyone can look at the numbers and know that they are not good, and I'm the first to say they are not good," Mattheus said. "When I break it down and I look at it, the one glaring thing is pitch location. Whether it's something mechanically or something mentally, it's not allowing me to get the ball where I want it. The bottom line is, I'm not making pitches.

"I would like to think my best attribute is mental toughness and being competitive. The only thing I can do is address the problem, and that's pitch location -- go out, compete my [behind] off and fix it."

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.