Markelle Fultz has been somewhat of an enigma. The Philadelphia Sixers combo guard has suffered through shooting issues that may have been related to a shoulder injury and recent reports even surfaced that he has been dealing with a wrist injury.

A recent explanation seemed to put Fultz’s condition in a clearer light. The 20-year-old is suffering from thoracic outlet syndrome and will be out for 3-6 weeks.

What, exactly, is thoracic outlet syndrome? The Mayo Clinic defines thoracic outlet syndrome as “a group of disorders that occur when blood vessels or nerves in the space between your collarbone and your first rib (thoracic outlet) are compressed.” The symptoms are shoulder and neck pain, as well as finger numbness.

Sports Illustrated interviewed Dr. Armin Tehrany, who explained the syndrome and claimed that it is hard to diagnose. 

“Thoracic outlet syndrome is a diagnosis involving the thoracic outlet becoming narrow. That’s an area … in the neck (near the shoulder) that can sometimes get compromised, especially in a case where someone is an over-head athlete — especially pitchers and very occasionally basketball players, as well. [It] can cause numbness, sometimes difficulty with cold sensation in that arm. But, regardless, it is something that has to be addressed.” 

Physical therapy is needed to treat the syndrome but it can also require surgery.

ESPN’s NBA insider Adrian Wojnarowski reported on Tuesday that Fultz will begin a rehab program in Los Angeles.