Sen. John McCain's doctors provided just the bare bones when they described his apparently unplanned operation last Friday.

Turns out, it was major surgery. Doctors had to remove a small piece of his skull to get at a blood clot.

"There's no such thing as minor in neurosurgery," Dr. Joseph Zabramski of Barrow Neurological Institute said in an interview Monday.

"Anything that we do involves significant risk."

There are several unanswered questions: What prompted the operation? How long will McCain's recovery last? What will doctors find in the tissue they removed?

McCain's confused questions last month for former FBI Director James Comey may also be seen in a different light now.

In an interview with CNN Monday, McCain's closest friend in the Senate, Lindsey Graham, let slip that McCain was becoming "forgetful" before the surgery last week.

Zabramski, who's worked as a neurosuregon for more than three decades, answered key questions about McCain's surgery -- a craniotomy that removed a 2-inch blood clot above his left eye.

Zabramski added the caveat that he doesn't know specifics about the senator's case beyond what his doctors have released publicly.

What's a craniotomy?

"In Sen. McCain's case, it was an incision in the eyebrow to expose a portion of the skull ... You have to make a hole in the skull to get to the surface of the brain. Then we can remove that blood clot without touching the brain tissue itself ... There's not a lot of pain associated with it."

"The biggest risk usually from removing a blood clot is that it can come back about 5 percent of the time" within 24 hours.

Zabramski said the operation could take up to an hour. "It can be a very, very simple operation through a very small hole."

What are symptoms of blood clot?

The brain "is this remarkably powerful organ, but it's very small. It fits in the palm of your hand. So a 2-inch round area could put some pressure on your brain.

"It's very possible there were no symptoms at all. I see patients not infrequently who come to the hospital who've had some sort of minor symptom, like a little headache ... We get a CT scan or an MRI scan and we see a blood clot."

Zabramski said McCain's surgery for skin cancer on his left temple in 2000 might have led doctors to include a CT or MRI in his annual physical.

McCain's doctors at Phoenix's Mayo Clinic said the surgery occurred after his annual physical.

"We don't like to leave blood clots in, because they can slowly enlarge," Zabramski said. "That's the biggest risk."

What comes next?

The tissue that doctors removed is being tested. Results are possible any day.

"What they're going to be looking for is whether this is just a blood clot or does it have any other elements in it, for example, is it a tumor that bled?

"A tumor has many blood vessels in it, and they can start to bleed and create a blood clot. But the fact that he had such a short course in the hospital would imply that wasn't the case."

How long could McCain's recovery be?

Doctors advised McCain to take this week off. Zabramski says the 80-year-old senator might need another week.

"You want to give somebody enough time that they're not fatigued anymore. Any time you have an operation, it's like being in a small accident. Your body goes into healing mode because you've injured the tissues. If there are no other issues, then two weeks would be a very reasonable recovery time."

The urgency to rush back to the Senate diminished Monday night, as the Senate Republicans' plan to repeal and replace Obamacare appeared doomed to failure.

Did blood clot cause McCain's confusion?

McCain's confused questions last month for former FBI Director James Comey led to speculation about his health. Could a blood clot pressing on his brain could have caused that behavior?

Zambramski played it down: "Is it possible, yes, but it's unlikely he could do so well for the next month."

But Sen. Lindsey Graham's told CNN Monday night that he believed the blood clot explained McCain's recent "forgetfulness."

"He'd been traveling a lot, we wrote it off that he was tired, but he was getting forgetful -- and you know he just wore himself out traveling all around the world. I'm glad they found out what I thought was the cause."

Could it be cancer?

CNN's Dr. Sanjay Gupta and other physicians have speculated that what doctors are calling a blood clot might be melanoma. McCain’s history of melanoma includes cancer surgery in 2000 on his left temple and cheek, not far from the site of the blood clot.

Zabramski was reading the Gupta story online when we arrived at his office.

"That's jumping through a lot of speculation to say that you think it's melanoma," he said. "Could it be? Yes. There's a small chance of that."