"We can only guess," says Dr. C. Thomas Vangsness Jr., professor of orthopedic surgery at the Keck School of Medicine of the University of Southern California.
Along with this recent procedure, Woods underwent surgery on the same knee in 2002 and prior to that in 1994 to have a benign tumor removed.
Woods originally injured his ACL when he was running at his home in Orlando in July 2007. He chose not to have surgery at that time and continued to play.
Dr. Misty Suri, a sports medicine orthopedic surgeon in the Ochsner Health System in New Orleans, says that sitting out the U.S. Open would not have saved Woods' knee from surgery. "Resting it doesn't cure that," Suri says of the ACL tear.
Woods is not the first professional athlete to push through a championship with this injury. Most recently, quarterback Philip Rivers of the San Diego Chargers played in the 2008 Superbowl with a torn ACL, and he had surgery three days later to repair it.
"Every sport has this as an injury," says Klapper.
Though golf may not aggravate the condition as much as football or basketball, Bronson still called Woods' decision to play in the U.S. Open "very bad." He says that Woods' ACL probably wasn't strong enough to support his knee.
Tears to the ACL usually occur after a traumatic event, such as running in Woods' case, but the repetitive motion of Woods' powerful swing could have aggravated this knee injury, Suri says.
On top of that, Woods may be experiencing pain from arthritis in his knee, Vangsness says. If that's the case, the hard cartilage that cushions the end of the bones is breaking down, causing pain and swelling as the bones begin to rub together.
Surgeons likely removed some of the fragments of cartilage from Woods' knee joint during the surgery in April, says Dr. James Gladstone, co-chief of sports medicine at Mount Sinai Medical Center in New York.
But arthritis progresses slowly, so "in terms of further damage to the knee, it's unlikely," Gladstone says about Woods's choice to play through the pain.
"Many elite athletes have a pretty good sense of their bodies … if there is a malfunction in the system, they are generally good at knowing that something isn't right," Suri says.
When Woods himself was asked if he might have re-injured his knee, he replied, "Maybe."
Though Suri tells most weekend athletes to refrain from playing through an injury, he recognizes that professional athletes have an additional set of concerns.
"The general public isn't getting paid millions of dollars for their athletic prowess," he says. "The decision for an elite athlete has entirely different implications than for the layperson."
It's too late for Woods to reverse his decision to play, but he can take a few steps now to nurse his knee before his return to the game.
"The best thing for Tiger to do is begin a water exercise program," says Klapper.
Klapper, who treated basketball great Wilt Chamberlain with the same regimen, recommends walking forward and backward in waist-high water for 20 minutes, three days a week. The water provides weightlessness to unload pressure on the joint and resistance to strengthen leg muscles surrounding the knee.
"It's a perfect double punch," he says.
Klapper performs about 500 surgeries a year, including hip and knee replacements. Many of his patients are in their late 30s and early 40s, suffering from damaged joints due to overexercising.