Tommy John was a pioneer in pitcher surgeries. He was the first pitcher to have his ulnar collateral ligament reconstructed in 1974 by Dr. Frank Gobe. Recently, I ran across a couple accounts of the injury and surgery that show some insights from the time.
First, Tommy John explains how the injury felt to him.
After the injury to his left elbow on July 17, 1974, Tommy John would wake up in his home in Yorba Linda, Calif., and the bones, he says, would go “pop and crack,” like giant dice rattling around. 
A few months later, the team doctor, Frank Jobe, performed what would become to be known eventually as Tommy John surgery.
The elbow did not heal, and Dr. Jobe went in and operated on September 25. It was worse than anybody expected. Totally shredded, Jobe had to cut a seven-inch strip of tendon from John’s right arm for use in sewing up the muscles in the torn left elbow. Even that may not work, Jobe said. Maybe the tissue would not be accepted, and if it weren’t , they’d have to do something else. Even if it did heal, Jobe said, he could make no promises about the ultimate strength of the arm. He called the operation “quite serious” and, on a scale of one to ten, he had it near the top “as one of the serious operations for a pitcher.”
Today the operation is fairly common with a good success rates, but it was cutting edge at the time. The operation did run into some issues when the ulnar nerve was injured and atrophy set in.
Dr. Frank Jobe took several inches of tendon out of Tommy’s right forearm and grafted it in the left elbow, but somewhere along the line the ulnar nerve began to drop out. This is a conduit of sensation known to the layman as the “crazy bone”, but it was like an unplugged electric wire so far as Tommy was concerned. The arm and hand began to atrophy and parts of the fingers [began] to mummify. 
Eventually Tommy got some good news and found out that the nerve would eventually heal.
Tommy had the usual cache of good news – bad news. The good news was the nerve regenerates itself. The bad news was, it does it at a rate of one inch per month. 
I asked Will Carroll of Sports Illustrated if the problem with the nerve exists today and he said:
Depending on which doctor does it, sometimes the ulnar nerve is moved during the operation.
Tommy was optimistic at the time of his return time to the majors.
“I’m the eternal optimist,” John says. “I have a couple of things in my favor. I have come back from elbow surgery before. I always keep my self in good shape. There’s no reason I can’t pitch until I’m 38, 39” ….. “If my arm stays healthy.
At the time it took Tommy 36 months to return pitching while today it takes a pitcher about 12 months. He exceeded his expectation of pitching until he was 39 by 7 years. In those seven years he pitched in 180 games, starting in 165 of them.
While Tommy John surgery seems fairly common today, at the time of the first one, it was experimental and success, of any kind, was not assured.
 Baseball Digest p72-73 “Tommy John: The Bionic Pitcher” Feb. 1976
 Sports Magazine Mar 1975 p66 – 77 “Sound Arms or Sore?”