Duncan, who went on the disabled list on July 22, had been batting .248 with 27 RBIs in part-time duty. He said the neck has been bothering him for a while but had been tolerable until after a game July 21, when he couldn't sleep and then woke up with stiffness.
Duncan, who acknowledged having tried to play through the pain before he consulted doctors, received a cortisone shot last week, and another one this week. The injections did not achieve the desired results, and he continued to have numbness in his left hand and weakness in his left arm.
The injury pinches a nerve that causes numbness down his right arm and affects his hand strength.
"The second shot I got better for like a day, and then it went right back to the way it was," Duncan said. "I was optimistic for a little while."
According to the St. Louis Post-Dispatch, the disk involved is between Duncan's C-5 and C-6 vetebrae.
"He had a little bit of relief the past day or two, but he's in a lot of pain," Cards manager Tony La Russa said before Friday's game against the Phillies. "He's dealt with a lot of pain and he's played a lot when he hasn't been right. He's caught a lot of [unnecessary heat] from people. It's not my favorite subject."
Duncan, 27, the son of pitching coach Dave Duncan, hit 22 homers in about half a season in 2006, and had 21 last year before being slowed by a sports hernia that required surgery in September. He's never really regained his stroke, and in May he was optioned to Triple-A Memphis to allow him a chance to work out his difficulties.
"I feel like I contributed a little bit," Duncan said. "But it certainly could have been better."
According to the Post-Dispatch, a prosthetic disk made of a combination of metal and steel will replace the herniated one in Duncan, a procedure that is believed to be a first for a professional athlete. It is a new surgery that could be a future alternative to fusing the disks, which tends to cost an athlete some range of motion.
"He's been in excruciating pain for the last 10 days," Dave Duncan told the Post-Dispatch.
Dave Duncan said doctors have projected a recovery time of three to six weeks, but added, "I would strongly urge against" his son trying to come back this season, according to the Post-Dispatch.
Information from The Associated Press was used in this report.