Mock, 27, has missed the last 65 games due to a right cervical disk injury. His only start for Washington this season was against the Mets on April 9, when he gave up two runs on four hits in 3 1/3 innings.
Nationals general manager Mike Rizzo said the move was a roster maintenance move.
"He's still rehabilitating from surgery, and we felt to get [Joel] Peralta up here, [Mock] was a likely candidate to go on the 60-day DL because of the neck surgery," Rizzo said. "He's not able to help us up here yet."