Ryan Zimmerman is scheduled to make a rehab assignment with Single-A Potomac on Tuesday and Wednesday.
Haren had his best start of the year and got things going at the plate. Meanwhile, Harper hit his ninth homer of the year in the Nats' 6-3 win.
Washington used early offense and a strong start by Dan Haren to win their third straight game over the Reds.
Davey Johnson said Zimmerman will likely join the team for their series at Pittsburgh beginning Friday, May 3.