The 6-foot-2, 318-pound defensive tackle from Fuquay-Varina (N.C.) says he has "a little tear" in his quad next to his knee. His doctor said it will heal on its own with rest.
"They don't want me to do any squatting or heavy lifting or running," Underwood said. "I'll have to find alternate ways to stay in shape, like swimming.
"I can walk on it just fine. It's just bending down on my knee that hurts."
Once the injury heals, Underwood's doctor informed him it won't cause any future problems.
Underwood believes the injury occurred approximately a year ago.
"We were hang cleaning and I dropped [the bar] on my knee," Underwood said. "I felt like I needed to play that year, because it was my year to basically get myself out there. So I played on it the whole season."
Underwood didn't have his knee looked at until this past Wednesday.
"Last Friday I woke up and I was on my way to workouts and I could hardly walk on it," Underwood said.
Underwood verbally committed to UNC a couple weeks ago.
"I'm pretty happy with my decision," Underwood said. "As far as the investigation goes, it's not going to change my mind either way."
Last weekend, Underwood visited UNC just to walk around the campus on his own to get accustomed to it.
"It felt good [knowing this is going to be my home in a year]," Underwood said. "I really liked the atmosphere and I liked the layout."
Underwood says he plans to make a visit with his mother before the season starts and then attend as many home games as he can.
UNC is recruiting Underwood as both a defensive tackle and offensive guard. He says there hasn't been any position discussion since he committed.
"I will play guard if that's what it boils down to," Underwood said. "But I want to play defense."