Knebel appeared to injure himself on a pitch to Francisco Lindor in the seventh inning on Sunday and exited the game after meeting with the athletic training staff. He is expected to have an MRI on Monday in Philadelphia to determine the extent of the injury.
Knebel missed 92 games with a similar injury last season with the Los Angeles Dodgers.
"We don't know if it's the same thing exactly," Phillies interim manager Rob Thomson said. "We'll know more after today. He couldn't lift his arm (on Sunday), which is kind of normal with this injury."
Knebel signed a one-year, $10 million contract with the Phillies in December, and was the team's closer before being moved into a setup role. He has a 3.43 ERA and 12 saves this season in 46 appearances.