Actually mate this is not uncommon for someone so young. Adolescent athletes under 21 can be more susceptible to ligament and other knee injuries. They require much more careful rehabilitation before coming back. Patterson is now 21 , so likely still in a susceptible age . If it is a reoccurrence of similar problems he needs much more careful rehab and strengthening work.
My missus is a physiotherapist (albeit not a specialist in sports) but as she always tells me, reoccurring injuries tend not to be a good sign for footballers.
Fortunately it sounds like it wasn't Pattersons knee and it was a hamstring injury so a completely unrelated injury to his previous one's. Hopefully just a slight hamstring strain