Nathan Karns will make his big league debut against Orioles young gun Kevin Gausman, above. (Nathan Denette / AP)

In need of a starting pitcher for Tuesday’s game against the Orioles, the Nationals reached down to Class AA Harrisburg for 2012 Nationals minor league pitcher of the year Nathan Karns, a hard-throwing right-hander who has made nine career starts above Class A.

With several starters signed as major league depth struggling at Class AAA, the Nationals chose Karns to replace lefty Ross Detwiler, who is on the disabled list with a slight oblique strain, in the hope his ability can overcome inexperience. Karns, named this winter by Baseball America as the No. 5 prospect in the Nationals’ farm system, has a 4.60 ERA with 55 strikeouts over 45 innings this year.

“It’ll be a good test,” Manager Davey Johnson said. “We’ll find out [if he’s ready]. He’s got a great arm.”

Karns, 25, arrives in the majors four years after the Nationals enticed him to sign with them out of Texas Tech with a signing bonus that far exceeded the recommended slot for a 12th-round pick. He missed all of 2011 after shoulder surgery, but last season he delivered on the promise his 6-foot-5 frame and mid-90s fastball portended, posting a 2.17 ERA at Class A.

The Nationals view Karns, or at least the allure of his potential, as one of their prime organizational successes. Their amateur scouts identified him at Texas Tech; their ownership approved the spending of the money it took to acquire him; their training staff rehabbed him from a major injury; their minor league coaches developed him. And now, Tuesday night, he will step on the rubber at Nationals Park against the Baltimore Orioles.

“Our farm system, that’s one of the reasons why we’ve been so good over the past two years,” third baseman Ryan Zimmerman said. “We have guys that can come up and make starts, guys that continually come through that pipeline and kind of help us continue to get better at this level. I’m excited to see him pitch.”

The Nationals added Karns to their 40-man roster this winter in order to protect him from the Rule 5 draft. He spent the first portion of spring training in major league camp, appearing in three games and allowing one earned run. When camp broke, Johnson said, the Nationals had decided he was one of their top three options behind their established starting five.

“I caught him a couple of times, and he’s got a good arm,” catcher Kurt Suzuki said. “Some good breaking balls. Obviously spring training is really early, he was working on some things. But he’s got a big-time arm. That’s for sure.”

Karns will oppose one of Baltimore’s top young pitchers in 22-year-old Kevin Gausman, whom the Orioles chose with the fourth overall pick last summer. Gausman allowed the Toronto Blue Jays four earned runs over five innings in a 12-6 loss last week in his major league debut. As Karns makes his debut, Gausman will start his second major league game.

Karns’s real challenge will come from the lineup he faces. The Orioles pounded 15 hits Monday against Gio Gonzalez and the Nationals’ bullpen. For the season, they are hitting .275/.332/.459 as a team. Karns will attack them with his 95-mph fastball, a curveball that acts like a slider and a solid changeup, ready or not.

“You try to make him as comfortable as possible,” Suzuki said. “We’ll obviously go over scouting reports with [pitching coach Steve McCatty] before the game. But when you go out there, it’s all about feel. You can do all the scouting reports that you want, but you gotta pitch to your strengths and that’s the bottom line. Try to keep him calm, he’ll be a little amped up out there. It’s his first start, he’s going to be nervous. But the more fun he has, I think the better he will be.”

The Nationals chose Karns over a pool of starters at Class AAA Syracuse that includes veterans Ross Ohlendorf and Chris Young. The Nationals signed Young to an incentive-laden minor league with the intent of making him their de facto sixth starter. Young, though, has punched up a 7.26 ERA in six starts. Johnson said the Nationals would have considered Ohlendorf, but he Saturday. Ohlendorf is 3-5 with a 4.83 ERA.

“Ohlendorf has been throwing good, but he’s not available,” Johnson said.

The Nationals sent left-handed reliever Xavier Cedeno back to Class AAA Syracuse to make room for Karns. Cedeno had been recalled Monday morning as insurance for a day.

>>> Second baseman Danny Espinosa’s right wrist responded well to swinging this morning and he could return to the Nationals’ lineup as soon as Wednesday. The swelling had decreased significantly, Espinosa said, and  “I didn’t get the same soreness I was getting before.” Espinosa was encouraged, but he said the most telling factor would be how his wrist reacts tomorrow.

“He’s probably going to be ready to come back on Wednesday,” Manager Davey Johnson said. “He said he could pinch hit. I told him during the game, ‘We don’t want to have a setback after the three days you had off.’ ”