Fisher said Long has a torn ACL and MCL in his right knee, an injury that will keep him out of the season finale against Seattle and require surgery.
The Rams and Fisher suspected the torn ACL immediately after the team's 23-13 win against Tampa Bay on Sunday but the addition of the MCL ailment means a longer road back than initially suspected.
"With respect to the fact that also the MCL is included, surgery will probably be delayed four to six weeks," Fisher said. "He takes very good care of himself. He knows and understands what's ahead. Obviously he is shocked right now but he's indicated to us that he'll be back and good to go."
Long suffered the injury on the Rams' third offensive play against the Buccaneers. The play seemed fairly innocuous as Long dropped into a pass set before his right knee buckled and he fell to the ground.
Fisher described the injury as "non contact," noting that Long lost his balance.
"It was kind of one of those weird things that happened," Fisher said.
Given the normal time required to recover from a torn ACL, the potential for Long's return in time for the start of the 2014 season would seem to be a bit of a long shot. Adding the four-to-six week waiting time for the MCL injury to heal and scar over could extend that.
Despite that window, Fisher said there's optimism that Long could return in time for the beginning of the regular season or soon thereafter. He pointed to the nature of the position as a reason Long could be back sooner than one might expect.
"I wouldn't say [it's] doubtful," Fisher said. "I think there's a difference between a skill player and interior line, especially an offensive lineman. The doctors were very optimistic, as he was."
Long signed a four-year, $34 million deal with the Rams as a free agent in March. He immediately jumped in at left tackle and started all 15 games for the team this season.
After some early hiccups, Long had been one of the team's most consistent blockers, especially after the Rams shifted to a run-heavy approach in Week 5.
With Long out, Fisher said Rodger Saffold, who filled in for Long at left tackle against Tampa Bay, will start this week against Seattle. Saffold was the team's starter at left tackle since 2010 before Long came to St. Louis.
Long's injury combined with Saffold's pending unrestricted free agent status has already had many wondering about the long-term future of the tackle position for the Rams.
Asked about the possibility that Long's injury would alter how the team views the NFL draft, Fisher said that's a conversation for another day and emphasized his faith in Long's return.
"We'll address that once we are finished with the season but I have confidence in him coming back," Fisher said. "He's an exceptional athlete, takes care of himself ... and he'll get himself back and ready to play."