Span was hurt in a collision with Atlanta Braves second baseman Dan Uggla in the eighth inning Friday night.
Span initially was stunned by the collision but he remained in the game after being checked on the field. Nationals manager Matt Williams said Span reported a mild headache after the game and had "mild symptoms" on Saturday.
Span had a serious concussion with the Twins in 2010, when he missed almost two months.
"Given his history, we thought it was certainly erring on the side of caution to go ahead and get him on the seven-day DL," Williams said.
"He had a pretty serious one before, so we just want to make sure on that."
Span remained at the team hotel on Saturday and may be examined again on Sunday to determine if he will accompany the team to Miami for a three-game road trip or return to Washington.
The Nationals also held right fielder Jayson Werth out of Saturday's lineup against the Braves due to cramping in his groin Friday night. Williams said Werth was available off the bench.
"Again, we think he's fine but at this point, looking at the long-term, we don't want to push him either," Williams said.
"We don't want him in a situation where he had to go first to third today or first to home and it be anything more than he feels right now. But if we get late in the ballgame, he can certainly win it for us."
Washington's makeshift outfield included Bryce Harper in center field, Kevin Frandsen in left field and Nate McLouth in right field.
Second baseman Anthony Rendon is filling Span's role as the leadoff hitter.
The Nationals recalled outfielder Steven Souza Jr. from Triple-A Syracuse. The team also selected right-hander Blake Treinen and optioned right-hander Blake Barrett to Syracuse.
Barrett, a rookie, has not allowed an earned run in 4 1-3 innings. Williams said he expects Barrett will be recalled soon. The team needed Treinen, a starter, to provide long relief if needed. Saturday was Treinen's scheduled day to start with Syracuse.
"He's done everything we've asked him to do, above and beyond," said Williams of Barrett. "The way our bullpen is situated now we needed a guy who could give us length. ... He's really pitched well for us and we expect him back soon."