"He could be here for three months or three days," general manager Ruben Amaro Jr. said. "We're not sure yet."
Sandberg started Tony Gwynn Jr. in center field for the third time in four games. Ben Revere, the incumbent centerfielder, sat for the fifth straight game. Revere's .581 OPS ranked 165th out of 174 qualified hitters.
The 26-year-old Revere, who will earn $1.95 million in 2014, has an option remaining. But the Phillies will not send him to the minors Saturday.
"Nope," Amaro said. "That hasn't been my thought. We have a lot of guys who have options here and haven't been playing all that well."
Domonic Brown entered Wednesday with a .578 OPS. That ranked 166th. Ruf could see time in left field if his stay is extended. One scenario - Marlon Byrd in center with Brown in right and Ruf in left - is not viable, Sandberg said. The manager said he "would be messing with a good thing" if he moved Byrd away from right. And Brown is more comfortable in left.
"He's kind of settled into left field," Sandberg said. "Nothing is set in stone. But I believe his coverage out there in left is better than in right."
Ruf played in 12 minor-league games as he recovered from the rib cage muscle strain suffered late in spring training. He homered Sunday and crushed two doubles Monday.
Ruf, 27, has 17 homers in 284 career at-bats.
"He's here because there's a need and an opening," Sandberg said. "Whether he needs some more at-bats remains to be seen. He's here as a possible threat in the meantime off the bench."