The Nationals selected Yean's contract from Triple-A Rochester on Sunday.
Washington designated outfielder Robert Hassell for assignment to clear room on the 40-man roster for Yean, who will be making his MLB debut when he gets into a game. The 25-year-old righty earned his first call-up after submitting a 3.69 ERA, 1.18 WHIP and 43:19 K:BB in 39 innings over 38 relief appearances for Rochester.