Riley, a 5-foot-11, 208-pound running back, didn't return to the game, which Kannapolis (N.C.) Brown won 25-22. The severity of the injury is still not known.
"I have a doctor's appointment [Monday] morning," Riley said. "I'm not 100-percent sure what's wrong with it. But it's definitely an MCL injury."
During Monday's doctor's appointment, Riley says he'll have an MRI and an X-ray.
"It sucks but I'm just trying to figure out everything and get back out there as soon as possible," Riley said.
Riley sprained the MCL (Medial Collateral Ligament) in the same knee ten games into last season. That injury forced him to miss three practices, but not any games.
"They said this one is a lot more serious [than last season's injury], but they're not sure yet" Riley said. "[Last season's injury] was a grade one. They said this one is at least a grade two."
Riley did, though, miss a playoff game with a concussion.
Despite the hardship, Riley is optimistic.
"I'm coming back [this season]," Riley said.
Riley hasn't spoke to the UNC coaches about the injury yet. He plans to call Troy Douglas, his primary recruiter, on Sunday afternoon.
Despite leaving the game early, Riley rushed for 105 yards on 22 carries, including a two-yard touchdown run.
Monday Update: Riley's MRI revealed a MCL tear, PCL tear and damage to his meniscus, according to Riley's mother, Tiffany.
"He's going to be out, we know, for the next four weeks," she said. "Anything after that we don't really know until the doctors at Carolina look at his MRI and see where they want to go with from there.
"We're going to come up there and have stuff done. His doctor worked closely with the Carolina doctors, so he's going to send everything up there, and then we're going to wait for what they say. I talked to the UNC coaches today."