According to Joe Reedy with the Cincinnati Enquirer, Sanu suffered the stress fracture in the fifth metatarsal in his left foot, which will require surgery on Monday after the Bengals critical Week 13 game in San Diego.
It's likely that the Bengals will place Sanu on season-ending Injured Reserve soon.
The Bengals entered the regular season with seven wide receivers and head coach Marvin Lewis "indicated" that Andrew Hawkins, who has been out for two weeks, could return to the slot this weekend against the Chargers.