Team spokesman Jeff Nelson said Still's injury occurred during a scrimmage Saturday. Still is scheduled to have surgery.
The redshirt freshman was expected to provide depth in a defensive tackle rotation that will already be without Chris Baker and Phil Taylor, who were kicked off the squad earlier this month for off-field issues.
Still also sat out last year with a left knee injury.
The 22nd-ranked Nittany Lions kick off the season Aug. 30 against Coastal Carolina.