After delivering that bad news, Mariucci also revealed linebacker Jamie Winborn will undergo surgery that will keep him out the remainder of the regular season.
The Niners hope that Winborn's surgery on Tuesday will make him available for the playoffs, but that obviously won't be the case with Williams, who will have surgery Thursday to repair damage to the medial collateral and anterior cruciate ligaments in his left knee that he suffered returning a kickoff on Sunday against the Dallas Cowboys.
"That's an unfortunate injury for Jimmy Williams," Mariucci said. "He was leading the NFC in punt returns and really becoming a productive player for us on kickoff returns, punt returns and our special teams units and that sort of thing. He'll be rehabbing for four or five months, and hopefully we'll have him back pretty much full speed for our first minicamp (in April)."
Rookie Eric Heitmann – the team's starting left guard – also underwent surgery to repair a broken right hand. He has damage to his third and fourth metacarpal, but Mariucci said that won't prevent Heitmann from returning to the starting lineup for Sunday's game against the Green Bay Packers, a showdown of first-place NFC teams.
"They'll put a couple of screws in there, but Eric will play ball," Mariucci said. "That's why they make casts. Hopefully, he'll be ready to go. We're going to get it fixed and just let 'er rip."
Winborn originally was expected to miss 4 to 6 weeks when he injured his left knee on Sept. 22 against the Washington Redskins. But he hasn't played since, and his slow recovery persuaded both Winborn and team doctors to try a surgical alternative.
"Here's the thing with Jamie," Mariucci said. "We feel that it's an operation that's going to recover in three weeks and we hope to have him in the playoffs. It's something that hopefully will be OK to participate and play come playoff time.
"Dr. Dillingham is going to be a busy guy."
Tailback Garrison Hearst suffered a hyper-extended knee against the Cowboys, but he's expected to play against the Packers. Among other new injuries to key players sustained in the Dallas game, linebacker Jeff Ulbrich has a concussion, tight end Eric Johnson has a left shoulder sprain and cornerback Ahmed Plummer has a groin injury.
All were "pretty sore" Monday, Mariucci said, and will be limited in practice during the week. But each is expected to be available against the Packers.