The center fielder and leadoff hitter has been out since July 7 because of back spasms that had plagued him the entire season.
His return -- coupled with that of Yunel Escobar from a hyperextended neck -- would give Washington its full projected Opening Day lineup for the first time this season. Injuries to Anthony Rendon, Jayson Werth, Ryan Zimmerman and more have disrupted the Nats' plans. Span's return would also give the team another consistent left-handed batter to go with Bryce Harper.
The Nationals (62-61) were heavily favored to win the National League East but have fallen 5½ games behind the surprising New York Mets.
The Nats had hoped to get Span back shortly after the All-Star break. However, after hitting, running and working in the outfield in early August, he stopped seeing progress with his back, which delayed his return. Span underwent an MRI, which showed no structural damage, when he was placed on the DL, and he saw a specialist in an effort to eliminate the injury.
He is batting .304 with five homers and 22 RBIs in 59 games this season, and he is 11-for-11 in steals.
Span also began the season on the DL after surgery to repair a right core muscle in March and surgery on a sports hernia in December.
The Nationals open a three-game series against the San Diego Padres on Tuesday.
The Associated Press contributed to this report.