In Saturday's 38-35 loss to the San Diego Chargers, he sustained his third in two seasons. Head coach Jim Harbaugh said Reid will visit a neurologist Tuesday to get a prognosis and develop a plan for Reid's recovery.
"Concerned? Yes," Harbaugh said Monday.
49ers players have dealt with concussions of varying degrees in 2014. Running back Frank Gore returned to the field Saturday after leaving the previous game in the second quarter, when a blow to the head area knocked him to the ground after trying to throw a block in Seattle.
Gore had his best game of the season Saturday, running for 158 yards and a touchdown.
Aldon Smith left Saturday's game in the second half with an apparent head injury and was examined in the locker room before returning late to play in overtime. Harbaugh indicated he expects Smith to be available Sunday against the Arizona Cardinals.
"They wouldn’t have put him back in the game if he had a concussion, no. He was evaluated without a concussion," Harbaugh said.
Last year, Reid left Week 2's game in Seattle and Week 10's loss to Carolina with concussions, but started the following week in both cases. But with the 49ers eliminated from postseason contention, there doesn't appear to be any reason to force the issue this week against Arizona.
Harbaugh did not provide any additional updates on injured players Monday, noting they will continue to get examined as the week goes on. But he did say he expects roster moves to be made before Sunday, which means the team's list of 16 players on injured reserve could grow.
Michael Crabtree (knee)
Bruce Ellington (hamstring)
Brandon Lloyd (groin)
Stevie Johnson (knee)
Bubba Ventrone (groin)
Ahmad Brooks (thumb)
Carlos Hyde (ankle)
Tramaine Brock (hamstring)
Eric Reid (concussion)
Aldon Smith (head)