MINNEAPOLIS - By now fans are used to Vikings players missing games because of an injury, but not so much when it’s the coach.

Head Coach Mike Zimmer traded the sidelines Thursday for surgery Wednesday night because of a detached retina. It’s third surgery in recent weeks. The Vikings said it was an emergency surgery.

"If you let it go untreated, it can lead to blindness,” said Dr. Minhee Cho, a HealthPartners ophthalmologist at Park Nicollet.

She hasn't treated Zimmer but knows a thing or two about what he’s facing. She tells KARE 11 a detached retina happens when the lining of the inner wall of the eye separates from that wall.

"The most important symptom is a peripheral dark curtain or shadow and it doesn’t move and if you see that, it’s a likely a sign of retinal detachment," she said.

Photo of a detached retina.

Other symptoms include seeing dark spots or flashing lights. Dr. Cho said it can be caused by age, trauma, family history and even near-sightedness. And after surgery?

"Typically we ask you not to read anything. Don’t do anything strenuous, don’t do activities that make your head move a lot for about a week,” she said.

And after that she usually wants her patients to take it easy. Best case scenario, she believes Zimmer is back in a couple weeks.

For fans who are used to seeing their purple this year take a pounding, they're focusing on the positive.

"We’re strong and we’re coming back,” said one fan during Thursday night's game against the Dallas Cowboys.