Solis was recalled from Triple-A Syracuse on Wednesday.
Solis will replace Matt Belisle (calf) in the Washington bullpen after the Belisle was placed on the disabled list with a calf strain. The left-hander has been impressive with the Chiefs so far this season, allowing only one run and striking out 14 batters in nine innings of work, although he'll likely slot into lower-leverage situations during his time with the Nationals.