The Nationals have announced that ace Stephen Strasburg has been placed on the 15-day DL (retroactive to June 16) with an upper back strain. A corresponding move will be made on Monday to fill Strasburg’s 25-man roster spot.

[Related: Updated Nationals depth chart at Roster Resource]

Strasburg was scratched from his last two starts due to the back problem, and given the backdate of his DL assignment, it’s possible he might not miss much more action. Still, the back strain is the latest in a rather lengthy series of nagging muscle injuries that have bothered Strasburg throughout his career, as he has missed time with oblique and neck strains in the past. (To say nothing, of course, of his Tommy John surgery in 2010.)

Tanner Roark replaced Strasburg in today’s scheduled start since Roark was on regular rest anyways, though Washington will need a pitching staff replacement later this week. Swingman Yusmeiro Petit is one option, and minor league arms Lucas Giolito and Austin Voth (as Chelsea Janes of the Washington Post notes) both have scheduled starts for Monday and Tuesday, respectively. However, there isn’t any sign that the Nats could call up either, Janes says. Of the two, Voth (at Triple-A) could be more directly ready than Giolito (Double-A), as Janes tweets that the Nationals want Giolito to polish his command before exposing the top prospect to Major League batters.

Strasburg already generated headlines this season by signing a seven-year, $175MM extension to remain in D.C., and he has been pitching like an ace worthy of such a contract. Strasburg has a 2.90 ERA, 11.4 K/9 and 4.92 K/BB rate over 93 innings, also notching a perfect 10-0 record over 14 starts. An extended absence is unquestionably a big loss for Washington, though the Nats have enjoyed strong pitching overall from Max Scherzer, Joe Ross, Gio Gonzalez and Roark.