"[This is] devastating for Sam," Fisher said.
The same injury sidelined Bradford in the middle of last season.
"Anyone that's gone through that procedure and the rehab understands and to have it happen again within the calendar year is very, very difficult," Fisher said. "We're thinking of Sam. I met with him this morning. He's left the building to spend time with his parents. I really can't give you any timetable as to when he's going to have this procedure done."
Bradford took what appeared to be a light hit from Cleveland's Armonty Bryant as he threw incomplete to tight end Jared Cook in the first quarter. Bradford shrugged Bryant off, but the defensive lineman made contact with the quarterback's left knee.
Bradford fell to the ground in pain as Rams doctors and trainers rushed to his side. He limped off the field under his own power but favored his left leg as he went to the locker room for further examination.
"It's just highly unusual," Fisher said of the injury. "There's a 1-in-100 chance that something like that would happen. It wasn't a high impact; ACL injuries, they are all different. This was just kind of a hyperextension with the impact coming from the outside. The knee was locked, and something has to give. Unfortunately the ACL gave."
Veteran Shaun Hill replaced Bradford, going 2-of-6 for 45 yards.
"This team is going to rally around Shaun, and this team is going to go play," Fisher said. "We have all the confidence in the world in Shaun."
Fisher shot down speculation that the Rams were working the phones to make a trade.
"It makes no sense to jump and react right now and try to fill the hole, whatever it costs," he said. "We're going to take our time and evaluate this. There's going to be some quarterbacks that are getting released, and there may or may not be some quarterbacks that have trade value. We just don't know. It's way too soon."
Bradford was making his second start of the preseason after returning from a season-ending torn left ACL suffered against Carolina on Oct. 20, 2013. Bradford went 4-of-9 for 77 yards before leaving Saturday's game.
Information from ESPN's Chris Mortensen and ESPN.com Rams reporter Nick Wagoner is included in this report.
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