Team officials provided no update after the game, but there was grave concern for Mauldin, who was unconscious on the field and strapped to a spinal board before he was carted off the field to a waiting ambulance.
Coach Todd Bowles and players said they saw "very little movement" from Mauldin, who was down for five minutes before he was transported.
"I'm very concerned right now, not knowing anything," Bowles said.
Mauldin got caught at the bottom of a pile on a fumble return in the fourth quarter of the Jets' 31-10 win at MetLife Stadium. Several bodies fell on his head, pinning it to the turf. He rose to his feet for a couple of seconds, but crumpled to the ground.
The stadium fell eerily silent as the training staff and emergency personnel tended to Mauldin. No fewer than 10 people stood over him, carefully moving his body to the spinal board.
Several teammates stood nearby, many of them on one knee, praying.
"I was really scared," wide receiver Brandon Marshall said. "I saw him get up and try to take a couple of steps and he just collapsed right in front of me. It was scary."
Safety Calvin Pryor said Mauldin was unresponsive.
"It's tough, man," said Pryor, who was also Mauldin's teammate at Louisville in 2013. "I went out there to check on him. He still had his eyes closed at the moment. I wasn't able to say anything or get a response. I'm just praying for the best at this moment."
Running back Chris Ivory said "it looked like his eyes kind of rolled into the back of his head, eyes closed. He didn't look to be doing so good at that time."
Mauldin, a third-round draft pick, is popular among teammates because of his upbeat personality and determination. He spent most of his childhood in Atlanta without his parents, bouncing around to at least a dozen foster homes with his brother.
On the night he was drafted, Mauldin was so overcome with emotion that he cried on a conference call with reporters.
Mauldin missed the last three preseason games with a sprained knee, but he was active for the opener as a situational pass-rusher and special-teams player.
"He was running around, doing some good things, and for him to go down like that, first, your heart goes out to him and his family," Bowles said. "After that, it goes out to his teammates. You just hope he's not seriously hurt."
For longtime Jets fans, the harrowing scene on the field may have triggered memories of another catastrophic injury -- Dennis Byrd's broken neck in 1992. He collided with teammate Scott Mersereau while trying to sack the quarterback, ending his career and leaving him partially paralyzed.