Robertson dove head first for the ball and Rios slid feet first, and it appeared Robertson's head hit Rios' knee. It also appeared in the slow-motion replay that one of Robertson's teeth flew out of his mouth; Robertson touched near his mouth after it happened.
The team hadn't made an announcement about the injury as of the sixth inning.
Robertson, who is 5-foot-8 and 170 pounds, remained down on the grass as a trainer and manager Ron Washington looked at him. He walked off under his own power.
Leonys Martin replaced him in center field.
A 28-year-old rookie, Robertson was playing in only his ninth major league game after he was recalled from Triple-A Round Rock earlier this week. He picked up his third career hit on Thursday, a single to lead off the game. SI WIRE: Giants starter Matt Cain exits game vs. Rockies with strained right hamstring