The Panthers pulled him out of practice. Davis went to the Emory Heart & Vascular Center in Atlanta for tests. The team sent his medical records to heart clinics around the country. Davis prayed. Eventually, the doctors' consensus was that Davis was in such good condition that he didn't need the surgery. He gets a stress test after every season to double-check.
"It was a tough time," he says. "You know, to be playing this game this long, playing at a high level, and knowing that -- just like that -- it was possibly over. That changes the way you look at things."
After all the surgeries, through all the pain, he took away an underlying message: His knee might fail again and again, but his heart is strong.
Thomas is the second member of Davis' lonely club. Last year Thomas tore his right ACL for the third time, and this year he became the first player besides Davis to make it back from that into the NFL. Now they talk and text about exercise tips and staying motivated.
"I didn't know how I was going to get through it," Thomas says. "I just knew the trauma. TD helped me get past that and look at the future. My daughter never really got to see me play before this year, you know? Now she has. I can tell her these stories and tell her, 'You never give up.'"
There is a cost to never giving up. Davis never believed it when old people told him they could feel it in their joints when it was about to rain. But now he'll feel the ache, look out the window and watch the clouds roll in.
His knee has cost him financially, too. He and the Panthers restructured his contract in 2012, when it wasn't clear whether he would play again. He gave up an $8 million bonus and signed for the veteran's minimum of $700,000. This year, he's playing for a $1.5 million base salary, with bonuses that could increase that to about $5.3 million.
But he didn't know whether the Panthers would take him back at all. After the third tear, he met with Panthers owner Jerry Richardson and then-general manager Marty Hurney to ask for another chance. Panthers coach Ron Rivera -- a tough guy, a linebacker on the Bears' Super Bowl champs under Mike Ditka -- says he has gotten choked up twice as the Panthers' coach. Once was when a disabled veteran spoke to the team for Veterans Day. The other was when Davis said he wanted to play again.
Athletes have this weird relationship with time. Davis spent nearly three years of his past recovering from his injuries. He spends countless lonely hours protecting his NFL future, building up his quads and hamstrings and hip muscles to save that little cord of fibers holding his right knee together. But all that time is intended to push time aside. He has worn a knee brace on the field ever since that first injury four years ago. But if he makes it through this season, he's taking it off next year. The brace makes him think about his knee on the field. In the moment, when he's running down a quarterback or checking a tight end, he can't think about how long it took to get back, or how long he has left.