WASHINGTON - Denard Span was reinstated from the 15-day disabled list Tuesday by the Washington Nationals, who hope the return of their centre fielder can fuel a playoff charge.

Span missed 40 games with a back injury. He will bat atop a lineup that operated for lengthy stretches this season without injured stars Jayson Werth, Ryan Zimmerman and Anthony Rendon.

"It's nice to have those guys back and be at full strength offensively," manager Matt Williams said. "It gives us a little bit of an option."

Span is batting .304 with five homers and 11 steals in 59 games this season. He also played in six rehabilitation games.

Washington entered a three-game series with San Diego on Tuesday looking to sustain the momentum created by 6-1 and 9-5 wins over Milwaukee this past weekend.

"Hopefully the offence continues to do what it's done the last couple (games) and we can make a little run here," Williams said.

Washington began the day trailing the New York Mets by 5 1/2 games in the NL East, but Span is confident the Nationals can make up the deficit.

"If I had money — which I do — I would put money on it," he said. "But there's no betting in baseball."

The 31-year-old Span intends to ease his way back into being an everyday player.

"I haven't played in a long time. My body still needs to get into shape," he said. "I've played one nine-inning game, and I was sore after that."

To make room for Span on the roster, Washington optioned right-hander Tanner Roark to Class A Potomac. Roark was a full-time starter last year, but this season he pitched out of the bullpen in 28 of his 34 games.

He will work in the minors as a starter, in part because rookie Joe Ross has thrown more innings than ever before and may ultimately be shut down for the season.

"He's in unchartered territory," Williams said of Ross. "It's start to start, day to day. If it's time to shut him down, then we've got somebody who's stretched out and ready to go."

Roark pitched 198 2-3 innings last year. This year, he's thrown 79 1-3. Thus, the trip to the minors, which will likely last the minimum 10 days.

"It prepares him for starting if we need to have him do that," Williams said. "It's about increasing his pitch count."