Strasburg was placed on the 10-day disabled list June 10 with right shoulder inflammation.
Strasburg is scheduled to start Friday night against the Atlanta Braves. The right-hander is 6-6 with a 3.46 ERA in 13 starts this season.
Zimmerman was originally placed on the 10-day disabled list on May 12 (retroactive to May 10) with a strained right oblique. He was then transferred to the 60-day DL on June 30.
Zimmerman, 33, is hitting .217 with 5 home runs and 16 RBIs in 33 games.
Matt Adams is starting at first base for Washington on Friday. Adams has batted .342 in his last 11 games and has 15 home runs on the season.
"He is going to play,'' Nationals skipper Dave Martinez said of Zimmerman. "I mean Matt Adams, he's pretty much killing right-handers right now. He deserves to play. I talked to Zim and he's going to come off the bench today.''
Pitchers Wander Suero and Trevor Gott were optioned to Triple-A Syracuse.
The Nationals, fresh off hosting the All-Star Game, are third in the NL East. Washington is 5½ games behind Philadelphia and five back of Atlanta.