Tom Brady said the trauma being experienced by Damar Hamlin and the Buffalo Bills puts NFL careers into perspective.
Bills safety Hamlin suffered an on-field cardiac arrest during Monday's game against the Cincinnati Bengals and was said by a representative on Tuesday to be "fighting" to recover.
Hamlin has been described as being in a critical condition at the University of Cincinnati Medical Center, with team-mates and opponents calling for prayers for the 24-year-old's recovery.
The incident occurred when Hamlin made a tackle on Bengals receiver Tee Higgins in the first quarter, and after briefly returning to his feet, he slumped to the ground.
He was attended to on the field before being taken away by ambulance, with the game suspended while the Bengals were 7-3 ahead with 5:58 remaining in the first period.
Seven-time Super Bowl winner Brady, still playing as a 45-year-old with the Tampa Bay Buccaneers, said: "We're praying for Damar and his family this morning in Tampa. Moments like this put into perspective what it means to play this game we love."
The quarterback great added: "Thankful for our communities in Cincinnati and Buffalo for the overwhelming level of care and support I know they’ll continue to provide."
Jordon Rooney, a marketing representative for Hamlin, told Good Morning America on Tuesday: "I can't speak specifically on his medical condition. I will say that he's fighting, he's a fighter. The family is in good spirits. We're honestly just taking it minute by minute, hour by hour."
He later released a statement from Hamlin's family that said that they wanted "to express our sincere gratitude for the love and support showed to Damar during this challenging time".
"We are deeply moved by the prayers, kind words, and donations from fans around the country," the family added.
The family statement praised the first responders and medics "who have provided exceptional care to Damar", adding: "We feel so blessed to be part of the Buffalo Bills organisation and to have their support. We also want to thank coach [Zac] Taylor and the Bengals for everything they've done.
"Your generosity and compassion mean the world to us. Please keep Damar in your prayers. We will release updates as soon as we have them."