New York Yankees closer Mariano Rivera sustained a possible career-ending injury on Thursday night before the Yankees game in Kansas City.

Rivera was chasing a fly ball at the Kauffman Stadium before the game when his left knee buckled, causing him to fall and tear his ACL. After falling to the ground, Rivera was carried away off the field by a cart.

After the game Rivera addressed reported, saying he tore his ACL (so badly it is considered broken) and his meniscus and doesn't know if he will ever be able to pitch again.

I don't know, at this point, I don't know, Rivera said.

But how serious is a torn ACL (Anterior Cruciate Ligament)?

According to the UCSF Medical Center, a tear to the ACL - one of the four major ligaments that connect bones within the knee joint - can be considered quite a serious injury, as the Anterior Cruciate Ligament provides 90 percent of stability within the knee joint. The ACL is located in the back of the femur and attaches to the front part of the tibia.

Injuries, ranging from mild to severe, can occur during a non-contact event when a person suddenly stops, moves, twists or cuts, frequently during sports and namely in football. Typically, surrounding ligaments like the medial collateral ligament (MCL) and the medial meniscus -- the second injury Marino Rivera sustained -- are injured in addition to the ACL about 50 percent of the time, according to the American Academy of Orthopaedic Surgeons.

Right after the moment of injury, a person can experience a pop in the knee, followed by sharp pain and swelling. Sometimes, however, a person does not feel pain and can continue with regular activity immediately afterwards while others can experience instability.

UCSF said that after a doctor conducts an X-ray to determine a torn ACL, reconstruction surgery is typically the next step in the road to recovery. However, only about 33 percent of people are able to continue normal activities without having surgery, instead opting for a strength program for six to 10 months. The reconstruction surgery, called endoscopic ACL reconstruction, option is typically chosen by athletes in order to repair the injury, which is done through a small incision. The process, which involves replacing the tear with a substitute graft made of tendon, is usually complete after four to six months of mandatory recovery.

The extent of Mariano Rivera's injury is by no means a fairy tale when it comes to reconstruction surgery, which requires a rigorous physical therapy process afterward. According to Web MD, when an injury to the ACL is severe, it can lead to other tears, damage to the cartilage and even osteoarthritis.  However, those who undergo reconstruction surgery often have an 82 to 95 percent success rate long term, according to AAOS, with only eight percent of people who experience recurrent instability or graft failure.

When it comes to Mariano Rivera, as well as New York Yankees fans, the fate of his career lies in the hands of doctors or surgeons, who ultimately determine the chances an athlete will be able to engage in sports again, if ever, following therapy.

View the video of the Mariano Rivera injury from Thursday night below.