Crabtree is undergoing additional tests to confirm the injury, the sources said.
Crabtree, 25, had his best season in 2012 for the NFC West champion 49ers, with 85 receptions for 1,105 yards and nine touchdowns. Crabtree was the 49ers' leading receiver during the regular season. Tight end Vernon Davis was second on the team with 548 yards and five touchdowns.
The 49ers have veterans Anquan Boldin, Mario Manningham and Kyle Williams on their depth chart at receiver. San Francisco also selected receiver Quinton Patton in the fourth round of the 2013 draft.