With Span landing on the disabled list, the Nationals called up Wilmer Difo to take his roster spot.
The rest of the Nationals' weekend rotation, however, has not yet been revealed.
The Nationals center fielder will fly to Dallas to see a specialist in the hopes of finding the root of his back spasm issues.
"I’ve tried to play through it for the last month. I don’t think I’m doing myself or the team any good."
Matt Williams said there are no plans "as of yet" for disabled list stints for either player.
The Nationals' starting center fielder and third baseman exited early against the Reds.
Injury updates on Bryce Harper, Denard Span, Yunel Escobar, Anthony Rendon, Ryan Zimmerman and the rest.
Escobar left Friday night's game after being hit in the hand with a pitch, and Span was a late scratch with back spasms.
The top of the Nationals order is starting to look like itself again, and that will help the offense.
"Just kind of hang out and do nothing. It's pretty brutal, actually," Zimmerman said.