Bradford reinjured the knee he had surgery on last season and St. Louis had four other starters hurt in a 33-14 win over the Cleveland Browns on Saturday night.
Bradford went down on his own after being hit by Browns defensive lineman Armonty Bryant in the first quarter. The Rams starting quarterback grabbed his knee and winced in pain before walking gingerly to the sideline with Rams coach Jeff Fisher watching every delicate step.
He was examined on the bench and then escorted to the locker room for further medical tests.
Following the game, Fisher thought Bradford hyperextended his knee, but will wait for more tests on Sunday.
''I'm very optimistic for his quick return,'' said Fisher, who didn't provide many details of the injury.
Fisher said he spoke with Bradford, who was not available for interviews.
The Rams are holding out hope Bradford's injury isn't serious.
The former No. 1 overall draft pick missed nine games last season after tearing his left anterior cruciate ligament and having surgery, and St. Louis was counting on him to bounce back and have a big year.
Bradford's injury wasn't the only one sustained on a rough night for the Rams. The most serious appeared to be to cornerback Trumaine Johnson, who was carted off with a towel draped over his head in the second quarter. Johnson and tackle Michael Brockers were injured on the same play, but Brockers walked off under his own power.
''It was difficult for our team there to have those things happen,'' Fisher said. ''I don't want the injuries to be the focus of this game. It's the third preseason game and we set our sights high and accomplished what we wanted to accomplish tonight.''
Fisher took a cautious approach with the 26-year-old Bradford in training camp and delayed the QB's exhibition debut until last week against Green Bay. Bradford played well, completing 9 of 12 passes for 101 yards, and Fisher had hoped to give him more time against the Browns.
But he didn't get past the Rams' opening possession. After driving St. Louis to the Cleveland 12, Bradford, who wears a brace on his knee, dropped back and was hit high by Bryant as he threw an incompletion. Bryant didn't wrap his arms around Bradford, who turned and fell.
''It appeared to be just a hyperextension of the same knee,'' Fisher said. ''We'll get you the information.''