"I can confirm now that Isaiah will be lost for the season," Fisher said. "He has torn his ACL which is very unfortunate as I said yesterday, as we suspected. He worked very, very hard to get in position to where he was a very productive special teamer for us and we had high hopes for him doing some things out of the backfield for us as well. So he'll have surgery in the next 10 days to two weeks which is typical with these injuries.
"He has a long road ahead of him but I'm confident that as he's matured over the last couple of years that he'll get back in good shape next year. We wish him the best through his rehab."
Pead suffered the injury while returning a kickoff after Green Bay scored on the opening drive of its 21-7 win Saturday.
With less than 10 minutes to go in the first quarter, Pead took the kick down the left sideline where fellow running back Benny Cunningham had opened a hole with a big block. Pead appeared to trip as he attempted a jump-cut, the field grabbed hold of his foot and his knee buckled.
Pead was not tackled on the play, and Fisher suspected that he was caught off guard by the sudden opening.
"It might have spooked him a little bit because when he came off the field, the doctors told him that he didn't hit anybody, wasn't tackled and he thought he was tackled," Fisher said. "It was just one of those non-contact, freak injuries that happen out there sometimes."
Pead was in the process of returning from a fractured finger suffered earlier in camp. He had a pin inserted into the finger and was returning the kick primarily using his left hand after sitting out the preseason opener against New Orleans.
The Rams originally selected Pead in the second round of the 2012 NFL draft. He's posted 75 rushing yards and 94 receiving yards in two seasons.