Karns, the Nats' No. 6 prospect, had his toughest outing of the spring Friday against the Cardinals, allowing seven runs (one earned) in two-thirds of an inning.
"[The injury] is a distraction right now for me," Karns said on Friday. "It's something that I feel every pitch. I'm out there not fully committed to the pitch. It's in the back of my mind. I don't know what's going on with it."
Karns has been dealing with tightness in his right leg since March 2.
Bill Ladson is a reporter for MLB.com and writes an MLBlog, All Nats All the time. He also could be found on Twitter @WashingNats. This story was not subject to the approval of Major League Baseball or its clubs.