Geiger, who will have arthroscopic surgery Wednesday, is the latest 76er to go down with a significant injury before the start of the season.
Stanley Roberts, signed last week to help the team's depleted front line, will start the season on the injured list while he recovers from shoulder surgery. Korleone Young, who was not expected to make the team, is recovering from a cervical sprain in his neck suffered two weeks ago when he was mugged.
Geiger averaged 13.5 points and 7.2 rebounds last season, second and third on the team in each category.
"I'm kind of scared, I admit that," Geiger said. "I'm trying not to get frustrated, but I definitely am. I know the last thing this team needs, with all the injuries we have, is for me to go down."
Geiger will have loose particles removed from his knee by team doctor Jack McPhilemy, who has been the busiest man in the organization in the preseason. An MRI on Tuesday showed no structural damage, said team spokesman Bill Bonsiewicz.
Rookie Todd MacCulloch, a second-round draft pick who has been impressive in exhibition games, is the likely replacement for Geiger at center.
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