SIOUX FALLSM S.D. -

The Minnesota Vikings dropped their record to 6-6 Thursday.

The Vikings kicked off Thursday Night Football without head coach Mike Zimmer on the sidelines.

Zimmer first reported having vision problems in late October, and has since had three surgeries on his right eye.

The night before the Vikings took on the Dallas Cowboys, Zimmer underwent emergency surgery to repair a detached retina.

"It’s a common misconception that injuries or activity will induce a retinal break or tear or detachment,” said vitreoretinal surgeon Dr. Ryan Geraets. “These things happen out of the blue with no warning."

Dr. Geraets says retinal detachment occurs when the gel in the back of the eye, called the vitreous, liquefies and separates from the retina.

It’s usually detected by symptoms of flashing lights or "floaters" in the eye.

The separation causes the retina to break or tear, and eventually detach.

If symptoms are not caught early enough, the effects could be severe.

“Blindness if it goes untreated, if the retina remains detached it’s not uncommon to actually lose the eye,” said Dr. Geraets.

Dr. Geraets says the fact that Zimmer’s most recent surgery was an emergency, likely means the center part of Zimmer’s retina was still attached, and has likely not yet taken away the center part of his vision.

Team officials have not said how much time Zimmer will miss.

Not knowing the particulars of his condition Dr. Geraets says recovery can last anywhere from two to eight weeks.

"If it’s a small break or tear that’s in the upper part of the retina, it could be a s little as two weeks and he's back to speed. If there are a number of breaks and tears all over, then he may be positioning face down for a week or more an unable to work for even 6 to 8 weeks, it really depends on the detachment"

There is no way to prevent retinal detachment, it’s important to undergo routine eye exams roughly every four to five years for those without any symptoms, and yearly for those over 40 and those who experience symptoms.

Proper eye heath is possible through a balanced diet, and eye protection.

Dr. Geraets says retinal detachments are fairly common, and account for about one-third of the surgical procedures in his office.