This is a difficult blow for a team that is looking to make strides in 2017 after winning just two games last season.
Smith set a career-high with 122 combined tackles in 2015 and immediately became a force in the Oakland Raiders defense after four years with the Seattle Seahawks. He also set a career-high in sacks (four) that year and followed up with 103 combined tackles in 2016.
The 49ers signed Smith to a five-year deal in the offseason, hoping he could provide stability to a defense that is still rebuilding and had to piece things together last season after NaVorro Bowman blew out his Achilles in Week 4 against the Dallas Cowboys.
The 2014 Super Bowl MVP is an anchor in the San Francisco front seven, but the 49ers have some defensive depth to fall back on.
Bowman, Ahmad Brooks and Elvis Dumervil will carry more of the load. Rapoport also noted rookie linebacker Reuben Foster could see his snaps increased.