Considering that he continued to play with the injury my guess is that it could be a meniscus injury of some sort.
I know they're different sports but I've seen basketball players, OKC's Russell Westbrook for instance, tear their meniscus and finish out the game as if there was nothing seriously wrong with their knee.
If it is a meniscus maybe the delayed reporting is based on whether they should repair the meniscus or completely remove it. From what I understand surgical repair puts you on the shelf immediately for a longer period of time whereas if you opt to completely remove the meniscus the return time is greatly reduced; however the player is more likely to have long term knee issues and will probably have his career shortened a bit.