J Pat Carter/AP

Jayson Werth is headed to the disabled list with a right hamstring strain, according to Nationals General Manager Mike Rizzo. Werth has dealt with the lingering leg injury for more than a week and after the outfielder suffered a setback Friday the Nationals opted to place him on the 15-day DL.

The Nationals will likely make the move following Friday’s game and backdate it to May 2, the last game Werth played. The muscle cramping first appeared April 29 in Atlanta when Werth struggled to get the right leg loose. During the game, it tightened up on him again; he left the game after he fouled a ball off his left ankle. He rested two days and when he played again felt the muscle tighten again. He hasn’t played since.

Rizzo said he expects Werth to come off the disabled list May 18, the first date he would be eligible.

All week, the Nationals and Werth believed the injury was simply cramping and not a pull. But as the injury lingered, an MRI was taken on Thursday. The initial results were clean. Werth was working out on Friday, doing leg curls and his right leg went in spasms again, Manager Davey Johnson said.

“Of the three grades, this would be a grade one-ish type of strain,” Rizzo said.

After doctor evaluations, the Nationals believe Werth’s ongoing cramping is attributed to dehydration from a stomach bug that has ailed him. Werth was prescribed antibiotics for the stomach bug Thursday.

“The hydration is the issue,” Rizzo said. “We think it could be just cramping because of the hydration. So we’re going to get that under control and see where he’s at and just be cautious with him.”

Two potential call-ups to replace Werth on the roster: Eury Perez, a speedy 22-year-old outfielder, is hitting .346 with seven stolen bases in 23 games for Class AAA Syracuse; and Jeff Kobernus, 24, is hitting .371 with 14 stolen bases in 31 games at Syracuse. Kobernus was Rule 5 draft pick returned to the Nationals from the Detroit Tigers late this spring. Outfielder Corey Brown seemed like the logical choice to be called up but he has been dealing with an oblique injury since April and the Nationals wanted a right-handed batter.

Perez, who is already on the 40-man roster, played in 13 games for the Nationals last season as a September call-up. The Nationals have an open spot on the 40-man roster, meaning Kobernus can be added without a corresponding move. The call-up is expected to join the team as soon as Saturday.

Perez wasn’t in Syracuse’s lineup on Friday evening and Kobernus was.

Kobernus is primarily a second baseman but the Tigers played him in the outfield this spring for the first time in his career and the Nationals continued have as well. Kobernus has played 13 games in left field and six games in center, added versatility that allows the organization to use his speed. He stole 42 bases in 53 chances last season for Class AA Harrisburg.

“It gives us versatility and more options onto how he can get on the roster and stay on the roster,” Rizzo said.