(Reed Saxon / AP)

The Nationals consider left-hander Ross Detwiler “questionable” to make his next start Monday because of discomfort in his lower back, Manager Davey Johnson said, further throwing the back of the rotation into uncertainty.

The Nationals have a clear immediate contingency in place if Detwiler cannot pitch. Dan Haren is eligible to come off the disabled list Monday, and after he threw a simulated game Wednesday, he would be on track to start Monday in Philadelphia. Rookie Taylor Jordan is currently listed as the Nationals’ probable starter for Tuesday.

“Nothing needs to be done at this time,” Johnson said. “It’s just a concern.”

Detwiler took two days off before throwing his bullpen session today, during which point the stiffness on the left side of his lower back flared up. Detwiler first felt soreness June 28 in a start against the Mets. Detwiler said the stiffness dissipated in his start Wednesday, but it hit him in his bullpen session. Detwiler will see team medical director Wiemi Douoguih tomorrow.

“He didn’t look the same in his last start, either,” Johnson said.

If Detwiler needs to miss a start, the all-star break would allow the Nationals to buy extra time. If he goes on the disabled list, he would be eligible to return July 19, the first day of the season’s second half.

Detwiler missed four starts earlier this season with an unrelated back ailment, a strained right oblique.

“Usually, back injuries, they’re debilitating,” Johnson said. “We need to just get by it and get him cured. When they’re there, there’s no such thing as just a small backache. If you get a spasm in your lower back, you got a problem – especially pitchers. That’s where they get their extension.”

Detwiler’s recurring health problems add to the Nationals’ questions in the back of their rotation. Behind their stellar top three starters, the Nationals at the moment would have to rely on some combination of a rookie with a dozen starts above Class AA, a left-hander with lingering back pain and a veteran with a 6.15 ERA who is coming off shoulder stiffness.

Jordan, Detwiler and Haren could end up being enough – in the first two starts of his career, Jordan allowed three earned runs in 10 innings. But the Nationals could also be in the market to trade for a starting pitcher before the July 31 trade deadline. One rival evaluator called a starting pitcher the Nationals’ “most obvious need” even before Detwiler’s setback.

Johnson called any discussion of trading for a starter “premature.”