The Nationals will promote pitcher Nate Karns, who will start against Kevin Gausman and the Orioles tomorrow, Adam Kilgore of the Washington Post notes (on Twitter). Karns will start in place of Ross Detwiler, who has a right oblique strain.
Karns ranked 99th on Keith Law's top 100 prospects list (Insider-only). He did not rank in Baseball America or MLB.com's lists. Law notes that Karns "was 93-97 as a starter with a yellow hammer of a curveball at 80-85 that he threw for more and more strikes as the season went on, meaning he could have two pitches that grade out at 70 on the 20-80 scale." Baseball America's Prospect Handbook lists Karns as the Nationals' fifth-best prospect and pegs him as a future mid-rotation starter or high-leverage reliever.
Karns, 25, had pitched this season at Double-A Harrisburg, posting a 4.60 ERA with 11.0 K/9 and 3.6 BB/9. He has not yet pitched at the Triple-A level, so his promotion qualifies as a bit of a surprise. It appears likely that the Nationals will send Karns down at some point, but if they don't, he will accumulate 125 days of service time this year. That means he might be eligible for arbitration as a Super Two player after the 2015 season, although the exact date that will establish the threshold of Super Two eligibility that season is unclear. He would be eligible for free agency after the 2019 season.