Zimmerman, who has been sidelined since July 7 with a strained rib cage, was in the lineup Tuesday batting seventh.
Taylor is batting .222 with seven home runs and 12 stolen bases in 66 games as a platoon player this season. His demotion opens playing time for top prospect Trea Turner, who has primarily played shortstop or second base but will start in center field Tuesday.
Zimmerman, 31, has been struggling through the worst season of his 12-year career. He entered Tuesday's contest with a career-low .221 average, 12 home runs and 38 RBIs.
Zimmerman recently completed a rehab assignment at Class A Potomac, where he batted .417 (5-for-12) with a homer and five RBIs. Although he hasn't hit for a high average, Zimmerman has been clutch -- 12 of his RBIs have given the Nationals the lead or been game winners.
Giolito, the Nationals' top overall prospect, made his third career start Sunday, allowing four runs in just 3 1/3 innings against the San Diego Padres. The 22-year-old Giolito has a 4.91 ERA in three starts with the Nationals this season.
Solis, who was sidelined 13 games because of an inflamed right knee, has a 2.43 ERA in 25 games for Washington.
The Associated Press contributed to this report.