Running back LeGarrette Blount, meanwhile, was ejected in the fourth quarter after leaving the bench area and shoving Elijah Lee after quarterback Matthew Stafford was hit as he was heading out of bounds. Blount, who was not in on the play, came over from the sideline and shoved Lee in the shoulder pads.
Slay's brain injury occurred on a third-and-1 in the third quarter when he went to tackle San Francisco fullback Kyle Juszczyk. Slay was on the ground for several minutes before getting up and walking to the team's medical tent at Levi's Stadium.
After being examined in the tent, Slay walked to the locker room without his helmet.
It's the second time in a week Slay had been examined for a concussion. He was cleared to return after going through testing in Monday's opener against the Jets.
Prior to the injury, Slay had five tackles.