Lopez was placed on the 15-day DL on Sunday due to knee soreness, Chelsea Janes of the Washington Post reports.
This may explain why Solis hasn't pitched since July 7, though his recent struggles may also be a factor. Control has been the only real problem this year, with the lefty allowing 16 walks in just 29.2 innings, albeit with 34 strikeouts, a 2.43 ERA and 1.21 WHIP. The Nationals still have a pair of southpaws, Oliver Perez and Matt Belisle, available in the bullpen.