Somewhat speculation on my part, but I wouldnt be surprised if he has a partially torn ACL or MCL and they’ve been waiting to see if it will re-attach on it’s own.
I say that as I have a fully torn ACL I’m waiting to have surgery on. Throughout the process, I learned a small tear in the ACL can fully heal in three months or less depending on the severity. If it’s completely torn, it won’t re-attach.
Given the time frame and an additional MRI, it at least seems like a plausible explanation.
I’m guessing it’s something like you describe or a partial meniscus tear.