Zimmerman has been hampered by a right oblique strain. He last played on Wednesday at San Diego.
"Last couple of days he felt the same, so we decided to just give him the 10 days," manager Dave Martinez said. "See if we can get this thing knocked out, calmed down."
The 34-year-old Reynolds has 11 seasons of major league experience, most recently playing 148 games for the Colorado Rockies last season. He played 10 games at Syracuse this year.
Reynolds hit .267 with 30 homers and 97 RBIs last year. Washington is his eighth major league team, and he has played 695 games at first base.
"He'll definitely help us. Veteran guy, he's got pop. He'll definitely get a chance to play," Martinez said, adding that Reynolds will face some upcoming left-handed pitchers.
Reynolds gets a $1.5 million, one-year contract and could earn $2 million in performances bonuses based on plate appearances.
"Wherever Davey needs me, I'll go," Reynolds said. "I'm ready to go, ready to help the team."
Zimmerman's DL stint is retroactive to Thursday. He said he would rather rest now than later in the season.
"Smart thing to do," he said.
Zimmerman is batting .412 (7-for-17) with a homer and three RBIs in five games in May. He is hitting .217 with five homers and 16 RBIs on the year.
Also Saturday, the Nationals transferred right-hander Joaquin Benoit to the 60-day disabled list with a right forearm strain.