It was only when pal Lisa Kudrow's neurologist brother discovered fluid leaking from his spine that medics realised just how serious his condition was.
In a candid new interview with The Hollywood Reporter, The Descendants star reveals, "We started doing these things called myelograms, where they shoot contrast into your system and you can see what's leaking out. I had a two-and-a-half-inch tear in the middle of my back and a half-inch tear in my neck.
"The doctors did these blood patches, where they tie you down to a bed, and you're awake because they have a long needle and need to know if they're touching your spinal cord, and they take blood out and shoot it directly into your spinal column to try to get the blood to coagulate in those spots. I did about 15 of those over 15 days. It's like getting a spinal tap every day, and you're awake... I thought I was going to die."
Clooney also tells the publication he struggled with the strong pain medication he was prescribed after doctors fixed the problem during a marathon surgery session on Christmas Day 2005.
He adds, "They'll hand you a giant tub of Vicodin, which is not a good drug for me; I had a lot of stomach pain and I really didn't like the high it gave me.
"Then there were other drugs. I was on morphine for a while, which created this horrible anxiety where I really thought I was in trouble."
Clooney reveals he finally turned to therapy: "I went to a pain-management guy whose idea was, 'You can't mourn for how you used to feel, because you're never going to feel that way again'."
He also admits he still struggles with the pain from his 2005 on-set injury, but it has become much more manageable.
The actor explains, "I've gone from where I can't function... to, 'I've got a bad headache'... My ears will literally pop and my head goes apes**t. But I'm scrappy."