On Monday, those fears were confirmed. Reid did indeed suffer a torn biceps in his right arm, an injury that will require season-ending surgery with six games to go in the season.
Reid acknowledged the injury on Twitter, joking that he didn't know whether he was more upset about not getting to play with his teammates anymore this season or not being able to curl in the weight room. He then said he was "down but not out."
Reid departed the team's locker room Sunday night with his right arm in a sling. He had an MRI revealing the injury on Monday afternoon.
Reid tied for the team lead with seven tackles in Sunday's 30-17 loss to the Patriots; he injured his arm on one of the game's final plays.
For the season, Reid had 62 tackles, 1 forced fumble and 1 interception. He has also joined quarterback Colin Kaepernick in kneeling during the national anthem before every game as a protest of racial inequality and oppression in the United States.
Early Monday afternoon, Niners coach Chip Kelly didn't yet have an update on Reid's status, but the team later confirmed the injury. Kelly did say second-year safety Jaquiski Tartt would likely be next up to replace Reid.
The timetable for Reid's return isn't yet known. He has one year remaining on his contract after the 49ers opted to use the fifth-year option on the former No. 18 pick in the 2013 NFL draft earlier this year.
Reid is the Niners' fourth defensive starter to suffer a season-ending injury, joining linebackers NaVorro Bowman and Ray-Ray Armstrong and defensive tackle Arik Armstead. Additionally, nose tackle Ian Williams suffered an ankle injury before training camp that ended his season before it began.