Bethea was at Stanford today having an MRI on the shoulder/chest area that he injured in Thursday night's loss to the Seattle Seahawks.
Bethea, who has started all seven games this season, had 44 combined tackles and a sack for the season. He has been to three Pro Bowls.
The move will make room for WR Jerome Simpson to join the active roster. Simpson is coming off of a six-game suspension and became eligible to rejoin the team for practices on Monday. However, with a shortened week, he was not activated for the team's game against the Seahawks.
The 49ers had until 1 PM on Friday to make a roster move to fit Simpson on the active roster.
Update:
Bethea's injury is actually a torn pectoral according to Matt Barrows of the Sacramento Bee.
League source: The 49ers place Antoine Bethea on season-ending IR to make room for WR Jerome Simpson.
— Matt Maiocco (@MaioccoCSN) October 23, 2015
Antoine Bethea injury, originally described as a shoulder, is actually a torn pectoral, per source.
— Matt Barrows (@mattbarrows) October 23, 2015