Nate Karns has not made his way to the Nationals’ clubhouse to the bullpen yet as rain falls at Nationals Park. Soon, though, he will appear and warm up for his major league debut, a start necessitated by Ross Detwiler landing on the disabled list and the Nationals’ desire to avoid roiling their bullpen.

The first time the Nationals needed to replace Detwiler, they still had hope he would avoid the DL and chose to start Zach Duke, with Craig Stammen backing him up. The use of their two long reliever left the bullpen unsettled for days before and after that game.

“What Davey [Johnson] and I talked about was, if Zach were to start or if Zach and Craig piggybacked each other, it messes up the bullpen for two or three days afterwards,” General Manager Mike Rizzo said. “This way, you get a guy that’s on rotation. He’s got good stuff. He’s on the 40-man roster. It’s just a cleaner fit for us, not only for the game today, but for the health of the bullpen.”

Johnson said he would keep a strict eye on Karns, probably keeping him to about 80 percent of his normal workload because of the draining nature of a major league debut. Duke, who has not pitched in a week, will back up Karns if the need for several innings in relief arises.

Because he feels Detwiler is improving and on track to start on Sunday, Johnson said the Nationals would likely call up a reliever Wednesday after Karns pitches, sending Karns back down. Johnson indicated the Nationals would call up right-hander Erik Davis, the lone pitcher available at Class AAA Syracuse on the 40-man roster. Despite a rocky outing last night, Davis is 1-2 with a 3.00 ERA and 27 strikeouts in 24 innings.

Rizzo was non-committal about Karns’s status. While Detwiler seems likely to start Tuesday, Rizzo was not 100 percent.

“Once we find out where Ross is at, we’ll make that determination,” Rizzo said.

The Nationals could soon have another option for the bullpen. Right-hander Christian Garcia, on the disabled list since the end of spring training, pitched in extended spring training and will leave for a rehab assignment shortly, Rizzo said.