Attorney Samuel Reef told The Boston Globe that the family of Rogerio Rodrigues is concerned for his well-being as it awaits more medical tests.
The 49-year-old Fall River, Mass., resident was a passenger in a minivan driven by his adult son when it collided with a car driven by Brady last Thursday in Boston. Rodrigues underwent emergency surgery and had been listed in serious condition.
Reef said Rodrigues took a turn for the worse over the weekend.
Brady wasn't injured in the accident and practiced with his teammates later that day. He said Monday on WEEI-AM that he was "scared out of my mind" after being involved in the crash.
"It was a crazy day. But I'll never forget that crash," Brady said. "I've been driving through green lights looking both ways now. ... Really, you just count your blessings.
"I was excited that I was able to go out there and come home and see my family right after that. That felt good. I got in the crash, I came home, I said, 'God, I'm just fortunate to be able to be here with my son and my wife.' That's really a blessing."
Police say Rodrigues' son ran a red light. The son maintains he had the green light.
Brady said after the accident that he left his car and saw a woman. He said they asked each other how they were doing and were OK.
"And I was just kind of looking around," Brady said. "I was looking in the other car to make sure they were OK."
Brady said he didn't know what to do because he was "just in shock."
"I was kind of sitting there in the car, looking around, trying to get my bearings," he said.
Brady didn't think about calling 911, he said, until a man suggested it.
"I was just thinking, 'How am I going to -- I've got to call (coach Bill) Belichick. I'm going to be late (for practice),'" he said. "And then once I got home, then it really hits you."
The Associated Press contributed to this report.