Tanner Roark. (Alex Brandon /Associated Press)

Rookie right-hander Tanner Roark will make his first major league start Saturday in Miami, according to Manager Davey Johnson, as he was chosen over Ross Ohlendorf and Stephen Strasburg to take the mound against the Marlins. Johnson had been weighing three options: starting Ohlendorf, replacing his spot in the rotation for at least one start with Roark, or skipping that start by starting Strasburg.

Johnson tabbed Roark because he has been wanting to give the rookie a chance to start. Roark, a power sinkerballer, has been impressive in his short time; he is 4-0 with a 1.19 ERA over 22 2/3 innings with 19 strikeouts and seven walks. Nationals minor league pitching coordinator Spin Williams thinks highly of Roark and his potential as a major league starter. Johnson also had previously expressed concerns with Ohlendorf’s endurance on the mound, and Strasburg has been dealing with a minor tightness in his hamstring.

“Ohlendorf’s been great, but I think that the young man has deserved an opportunity to start a game or two before the end of the year,” Johnson said. Ohlendorf will remain on the same schedule as Roark in the event of any hiccups.

Strasburg has been dealing with minor hamstring tightness — although it’s unclear which leg — for the past “couple starts,” Johnson said. The Nationals didn’t want to risk anything and take the extra day away from Strasburg. Johnson said the right-hander is expected to make his next scheduled start Sunday.

The Nationals have won all five of Ohlendorf’s starts. He has served his role well as a long reliever and fill-in starter, posting a 2.98 ERA over 48 1/3 innings. But since he returned from the disabled list on Aug. 21 following right shoulder inflammation, he has pitched decently but labored. In his past three starts since joining the rotation following Taylor Jordan’s shutdown, Ohlendorf has a 5.65 ERA over 14 1/3 innings. He purposely fluctuates the velocity of his fastball, but so much so that Johnson once feared endurance issues. Johnson believes Ohlendorf is healthy.

“His differential is really huge,” Johnson said. “Usually you subtract away and add in. He’s liable to do it any which way. But he’s done a great job. I have no fault in him. This is more. … I don’t think he’s out of gas or anything like that. This is more giving Tanner Roark an opportunity to start a game up here in the big leagues, see how he does. He’s been unbelievably good out of the ‘pen.”