While Kiwanuka did not miss a practice this week in preparation for Sunday night's game against the Chicago Bears, he complained about stiffness in his neck on Thursday and Friday.
Kiwanuka was examined by team surgeon Dr. Russell Warren and spine specialist Dr. Frank Camissa at the Hospital for Special Surgery in New York City after Friday's workout and a bulging cervical disc was discovered.
"It's disappointing. It just feels like a stiff neck," Kiwanuka said. "The pain is not intolerable but I want to make sure that I will be OK down the line. I have full trust in the medical staff and the trainers at the Giants. I'm going to do everything that is asked of me so when I'm cleared I'll be back and ready to go."
Giants spokesman Pat Hanlon said there is no timetable for Kiwanuka's return. He added the return and treatment vary from case to case.
Doctors will continue to monitor the disc to see if it quiets down.
"Obviously, Kiwi is a big part of our plans. He's had a very good season," coach Tom Coughlin said Friday night. "He's very flexible. He's a versatile athlete who can be used in a lot of different ways. So now we have to regroup and try to figure out how we're going to utilize our personnel for this game."
Coughlin said it was difficult to get the news on Kiwanuka so late in the week.
"It is, because you put stock in the fact that it's not going to happen," he said.
Kiwanuka had a team-high four sacks and 11 tackles this season. While he alternates positions on the defensive line, his athleticism has allowed the team to use him at linebacker.
Trainer Ronnie Barnes said that Kiwanuka came to work on Thursday with a stiff neck. He was treated, practiced, and went to New York for an MRI on Thursday night.
Kiwanuka had some stiffness in his neck but otherwise felt OK. He went through practice and intentionally avoided contact with his head and neck.
After practice, he was sent in to the Hospital for Special Surgery for a CT scan.
Linebacker Keith Bulluck is doubtful for New York with a toe injury.
"I've only missed two games in my career," Bulluck told the Daily News on Friday. "To miss a game or miss a practice, it kind of means something to me."
The Associated Press contributed to this report.