Justin Parker's body told him that enough was enough.
So on Monday, the Tigers' junior linebacker informed head coach Dabo Swinney of his decision: he's going to call it quits on his football career.
Parker, a highly-touted linebacker from Beaufort, S.C., saw spot action and special teams duty for the Tigers during his first two seasons. Then he took a redshirt year last fall as he recovered from groin surgery.
Parker was working his way back into the Tigers' linebacker mix this spring, but has continued to struggle with issues with his left knee that date back to high school.
Swinney said that Parker's decision came as a surprise to him - "literally out of left field."
“He said that he just can't do it anymore with his left knee," Swinney said. "He's kept a lot to himself. He's been playing with pain. He said he wakes up with pain every day.
"He's been playing a long time. It's something he's dealt with since high school, with his left knee. He's just got bone on bone. He's just been doing a lot of thinking. He said he can't do it anymore. The last thing I want is a guy taking Advil every day, just to get through the day."
Swinney described Parker as "one of the most respected guys on this team."
"There has never been one issue with him, has handled himself the right way," he said. "I hate it for him. He's a very mature young man, and I appreciate his efforts and all that he's done. I hate to see his career cut short."
Parker will stay on with the team as a student coach and his scholarship will be honored, but will not count toward Clemson's NCAA limit of 85.