Buffalo Bills safety Damar Hamlin is beginning to awaken as he shows "substantial improvement" in his recovery from going into cardiac arrest and collapsing on the field during Monday's game, doctors at the University of Cincinnati Medical Center said Thursday.

One of the doctors, Timothy Pritts, said Hamlin is still considered critically ill and that significant progress is needed. Hamlin's neurological condition appears to be intact, Pritts said.

Hamlin, 24, is able to move his hands and feet, the doctors said. He is unable to speak but was able to communicate in writing, Pritts said. Hamlin asked whether the Bills won Monday's game against the Cincinnati Bengals -- a contest that was postponed after his collapse.

"The answer is yes, Damar, you won. You won the game of life," Pritts said.

Asking about the game was a sign to doctors that "not only the lights are on, but he's home" and was a gratifying sign.

CPR was administered to Hamlin on the field Monday night for multiple minutes after he collapsed following his tackle of Bengals wide receiver Tee Higgins. Hamlin received oxygen, according to the ESPN broadcast, as he was placed in the ambulance and taken off the field. He was driven to the hospital, where he has remained.

Dr. William Knight IV credited the quick medical response with saving Hamlin's life. He said a physician was by Hamlin's side within a minute of him collapsing and recognized that the defensive back did not have a pulse. Knight said Hamlin required CPR and resuscitation on the field.

There is no definitive answer to what caused Hamlin's cardiac arrest.

"It's been a long and difficult road for the last three days," Knight said. "... He has made a pretty remarkable improvement."

Pritts said neurological signs of improvement began Wednesday night as Hamlin gradually woke up, with the rest of his body healing. The doctors said it is too early to tell how Hamlin is doing emotionally in part because he is still on a ventilator. Doctors also have not fully assessed his speech and other functions, in part because he is under sedation to accommodate the breathing tube.

"When we talk about neurologically intact, it's a very gross term of big motor movements and following commands," Knight said. "When we talk about the finer things that make us human -- cognition, emotion, speech, language, etc. -- we're looking forward to learning more about that soon."

For Hamlin to be upgraded from critical to stable condition, he will need to be taken off a breathing tube and show improved neurological and breathing functions, the doctors said. They would then like to get him home and see about potential steps for the future.

It's also too early to say whether Hamlin could return to football after undergoing rehabilitation, Knight said.

Hamlin's father, Mario, released a statement Thursday through family friend Jordon Rooney thanking everyone for supporting Hamlin and his Chasing M's foundation, which has raised more than $7.3 million on GoFundMe. Mario Hamlin, who serves as the foundation's executive director, asked that further donations be sent to the Chasing M's website. He also asked that, in support of first responders, donations be sent to the University of Cincinnati Medical Center trauma center or by buying first responders and trauma centers lunch in support of the care that his son has received.

The NFL announced Tuesday that the Bills-Bengals game will not resume this week, and the league is undecided on playing the game at a later date. The NFL has not made any changes to the Week 18 regular-season schedule.

The Bills began their practice week Wednesday with a walk-through and team meetings in preparation to host the New England Patriots on Sunday. They are expected to hold their first full practice of the week Thursday.

They made a roster transaction to help the safety group that will be without Hamlin, signing Jared Mayden off the New York Jets' practice squad and releasing veteran cornerback Xavier Rhodes. Mayden spent time on the Bills' practice squad earlier this season.

ESPN's Alaina Getzenberg and The Associated Press contributed to this report.