On May 6 at Nationals Park, in the first starting assignment of his career, Treinen suffered the loss in a matchup with Clayton Kershaw despite blanking the Dodgers through 5.0 innings. Thanks to a sixth-inning fielding error, Treinen allowed only three unearned runs on seven singles in 5.0-plus innings (0 BB, 2 SO) as the Dodgers prevailed, 8-3.
Treinen, in 3 April relief appearances, registered a 1.35 ERA in 6.2 innings of work during his first stint with the Nationals.
In six starts for Triple-A Syracuse this season, Treinen threw 23.0 innings and compiled a 1.96 ERA. He allowed just five earned runs, walked five and struck out 17.
The hard-throwing right-hander was acquired by the Nationals, along with right-hander A.J. Cole and left-handed reliever Ian Krol, from the Athletics in the three-team trade in Jan., 2013, that sent outfielder Michael Morse to the Seattle Mariners. Treinen was a seventh-round draft pick of the Oakland Athletics in the 2011 First-Year Player Draft (No. 226 overall).