Bradford suffered the injury as he scrambled toward the sideline on a second-down play with 5:27 to play in their 30-15 defeat by the Panthers.
Carolina safety Mike Mitchell converged on Bradford but the injury appears to have been caused when he landed awkwardly on his knee after stepping out of bounds.
Bradford went for a scan straight after the game and the Rams later confirmed that initial reports suggest were correct and he had torn his ACL, an injury that has definitely ended his season.
"I'm not going to deal with hypotheticals right now until I find out what really is going on with him," coach Jeff Fisher said straight after the defeat.
"It makes no sense to rush to a conclusion until we know. We'll get a medical and then we'll go from there. He was in significant pain on the sideline."
With the Rams losing Bradford to injured reserve, they need to move quickly for possible replacements, having only back-up Kellen Clemens at the position on their roster or practice squad.
Among the names being considered, according to NFL Network's Mike Silver, is Tim Tebow who is without a team after being released by the New England Patriots at the end of August.
St Louis will work out a number of quarterbacks in the next few days, with Tebow not looking like the favoured option.
It could also be bad news for Bradford, with the Rams owning two first round picks in next year's Draft, and with a number of QB options coming out of college.
Bradford has been steady if unspectacular in St Louis, admittedly without a decent stock of weapons to use, but it remains to be seen if the team remain set on him as their franchise quarterback going forward.