I did a little reading. The patella is the knee cap and the tendon connects the knee cap to the bone below it. Patellar tendonitis is common for people who do a lot of jumping. It's usually just an inflammation like would happen with anything that is overused.
I mention this because I see a lot of comparisons with other people. None of them that I know of have had the surgery necessary here. For what it's worth, here's a quote:
Is surgery ever needed for patellar tendonitis?
Exceedingly rarely. On some occasions, patients have persistent patellar tendonitis symptoms despite the treatment mentioned above. In these patients with chronic patellar tendonitis, who have tried the above treatments for at least a year, surgery can be considered. Surgery is usually considered if a portion of the tendon can be seen as degenerative on the MRI.
Frankly, I'm not sure what it means as to Hassan's future. I'm bothered that this is in a guy so young. Didn't anyone know about this before he was drafted? It seems very odd but maybe Hassan kept his mouth shut.