ESPN.com reported he separated a collarbone while trying to make a diving catch during the second quarter.
Coach Jeff Fisher said Friday he would not have an official injury report until next week, but he was sure Amendola would at least miss the Rams' next game Oct. 14 against Miami.
Amendola has caught 32 passes -- tied for the top spot in the NFL -- for 395 yards and two touchdowns.
St. Louis has a 3-2 record, having won one more game this year than during the entire 2011 season.
Swim Week Miami Beach 2014 [PHOTOS]