49ers safety Antoine Bethea was placed on injured reserve Friday after suffering a pectoral tear in his upper right chest in a 20-3 loss to the Seahawks.

The 31-year-old Bethea underwent an MRI exam also on Friday that revealed the extent of the injury.

Bethea appeared to injure himself in the second quarter on Thursday while committing a defensive infraction by yanking on Doug Baldwin's helmet and was called for a facemask penalty, ESPN reported.

Baldwin had just caught a pass and was running past Bethea when the latter reached back with his right arm to corral the former around the shoulders.

Bethea, who was voted team MVP last season and earned his third trip to the Pro Bowl in January, had 32 tackles and a sack so far this season.

He started his first 23 games after joining the 49ers as a free agent last season, when he also tied a career high of four interceptions. It was also last season when Bethea posted his first career touchdown, and had a career-best 10 passes defended along with 72 tackles, a sack and a forced fumble.

He previously spent eight seasons with the Colts.

Rookie Jaquiski Tartt, the 49ers​' second-round draft pick from Samford, is expected to replace Bethea. Tartt suffered a leg contusion Thursday night and has 21 tackles and a sack thus far.

Due $4.5 million, Berthea may be viewed as a luxury if Tartt impresses in the rest of the season.

The ​49ers will fill the vacated roster spot by activating receiver Jerome Simpson from the reserve/suspended list.

Simpson was serving an NFL-mandated six-game suspension to begin the season, but the Niners received a roster exemption for him before the Seahawks game.