The other possibilities include a sprain of the medial collateral ligament or simply a
tear of some scar tissue
from your previous surgery.
The problem with a post surgical knee, is that an MRI may not be that helpful. It may not be possible to differentiate between post-surgical changes and a new tear. Usually, these injuries ar treated clinically. Rest, ice, bracing, NSAIDs, and time. If the pain resolves, then no further action is taken.