(Reed Saxon / AP)

Since Sunday, the day after the bullpen session he threw in anticipation of a Tuesday start he couldn’t make because of a lingering right oblique strain, Ross Detwiler has been shut down and resting. He threw a clean bullpen on Saturday with no issues but the following day, as he fielded grounders from the mound during practice, he again felt tightness in his side.

The Nationals called up rookie right-hander Nate Karns from Class AA Harrisburg and he provided a stopgap Tuesday, tossing four 4 1/3 innings and allowing three runs.

The combination of Detwiler’s setback and Karns’s performance prompted the Nationals to hold Detwiler back further and keep Karns for another start on Sunday in Atlanta. Detwiler last pitched on May 15 and was placed on the disabled list retroactive to May 16.

“I was kinda doing the [fielding drills] and I felt something there,” Detwiler said. “We kinda talked about it. It’s too early in the season to push through it and don’t want to deal with this for the next four or five months.”

Detwiler said he has solicited advice from teammates who have suffered the injury. Ian Desmond missed 25 games with it last season, and Adam LaRoche and Mark DeRosa have dealt with it, too. According to the Los Angeles Dodgers’s medical director Stan Conte, who has studied decades worth of oblique injuries and other injuries across the sport, pitchers miss an average of 35 days with oblique injuries and position players about 26 days.

Detwiler will continue to rest (“If [trainer] Lee [Kuntz] catches me do anything I think I’m in trouble,” he said) and even travel with the team to Atlanta. Trainers are monitoring his side daily. Since this is the first time he has dealt with an injury of this kind, he is unsure of any timetable and whether he would need a rehab assignment before returning.

“You just try to talk to everybody that’s had it in the past,” he said. “A lot of hitters kinda get it every once in a while. See what they do, how their rehab goes, try to adapt that. I asked everybody if they’d ever had it, and if they did, I asked them what they did.”

>>> Bryce Harper’s condition remains the same. He is dealing with bursitis in his left knee, which remains swollen. He is out of Wednesday’s lineup, the third straight game he will miss since he aggravated the injury Sunday. The Nationals are hopeful that Harper can return as soon as the inflammation improves. They said injury isn’t structural.

Harper has tried to take batting practice and Manager Davey Johnson has put the brakes on that. He said he hopes Harper could be available on Friday in Atlanta. “I don’t have a lot of high hopes but I’m hopeful,” he said. “… However long that takes is however long that takes.”

>>> First base coach Tony Tarasco has been excused from the team since Sunday as he tends to his wife and the birth of their son. They encountered a minor complication, according to Johnson, and Tarasco will remain away for the team for a few more days. Johnson said the baby boy is doing fine.

Hitting coach Rick Eckstein has been coaching first base in Tarasco’s absence.