NFL Network's Jason La Canfora reports that Jenkins might have a torn anterior cruciate ligament, according to a league source.
Ryan didn't want to further speculate on the future of Jenkins, one of the Jets' defensive stars, until he had an MRI exam. Jenkins' leg was heavily wrapped after the game, and he left the stadium on crutches.
"We'll know much more about it (Monday)," Ryan said. "Unfortunately, I think it's probably severe. I'm not sure, though."
Jenkins tackled Bills running back Fred Jackson on a 4-yard run with just over 5 minutes left before halftime and went down, grimacing while laying on his back. Jenkins ripped off his helmet and stayed on the turf on his back for several minutes as trainers checked on him.
Jenkins slowly walked off the field to the sideline, where he was further examined before gingerly walking under his own power to the locker room. Jenkins, in his second season with the Jets, is considered a centerpiece of Ryan's aggressive defense.
"It's a huge blow," defensive end Shaun Ellis said. "That's our guy. Now guys are just going to have to step up. ... Guys are going to have to step up across the board and fill in his shoes, and those are big shoes to fill."
Right tackle Damien Woody left midway through the third quarter with an injured right knee, but he said after the game that he'd be fine.
"I don't even worry about that type of stuff," he said.
The Associated Press contributed to this report.