(Reed Saxon / AP)

Left-handed starter Ross Detwiler could be sidelined for the remainder of the season with a herniated disk in his lower back, Manager Davey Johnson said.

Detwiler has been on the disabled list since July 4, after he made a handful of starts pitching through spasms in the left side of his lower back. Detwiler visited specialist  Robert Watkins Sr., a renowned back surgeon based in California, last month for a second opinion. Watkins diagnosed the herniated disk and prescribed a month of rest and anti-inflammatory medication.

By that point, Detwiler will not have pitched in about two months. The Nationals would place Detwiler on a full throwing program to rebuild his arm strength, “almost like you go through spring training again,” Johnson said.

The Nationals are hopeful Detwiler can return in September, regardless of whether they have clawed back into contention.  They also want to ensure the rest works to rid Detwiler of the herniated disc and give him a clean bill of health entering 2014. But Johnson said there may not be time for him to complete the throwing program and pitch in the majors.

“We’ll try to bring him back before the year is over to make sure he’s healthy, to make sure nothing further has to be done with him,” Johnson said. “That’s the main thing, that he can be healthy once he gets the inflammation out of there. … It’s really going to be a big concern of our doctors seeing how he does when he does come back.”

Detwiler’s season has been derailed by injury. He carried a 2.53 ERA into his eighth start of the season and then strained his oblique. After almost a month on the disabled list, Detwiler returned, but was not the same. In five starts, pitching through obvious discomfort in his back, Detwiler went 0-3 with a 6.31 ERA.

“I just felt like there was something off and I was trying to fight through it, which could’ve made it worse,” Detwiler said after he returned from his visit to Watkins. “So it’s something I kind of have to sit back and say, ‘If this ever happens again, you just can’t try to fight through something like that.’ I think we took the right step” visiting Watkins.

Rookie Taylor Jordan has been impressive filling in for Detwiler, punching up a 3.31 ERA in six starts after coming straight from Class AA Harrisburg. But Jordan will be shut down soon, once he reaches a team-mandated innings cap. Jordan, who underwent Tommy John surgery in 2011, likely only has two or three more starts remaining, including Sunday against the Brewers.

The top candidate to replace Detwiler would be Ross Ohlendorf, who faces his own minor health concerns. Ohlendorf left a relief appearance Wednesday after only 11 pitches because his velocity was down. He played catch this afternoon and felt much better, he said. Ohlendorf, who has a 1.87 ERA in a pleasant surprise of a season, will not pitch in a game until at least Monday. But he has no concern he’ll be fine.