Dodgers center fielder
Kemp, who has been working out with the team after rehabbing recently at Camelback Ranch in Glendale, Ariz., was expected to return to the active roster Friday.
"We've hit a little bit of a snag because he felt just a little something today still," Mattingly said.
The medical staff will watch Kemp run Sunday and assess his status.
"Three days, two days, seven days, I don't know that answer," Mattingly said of Kemp's possible return. "But we do know he's not 100%."
Kemp was hitting .251 with two home runs, 17 runs batted in and seven stolen bases before straining his hamstring.