^^^
From the little information out there about the tests that were being done post surgery medical people were speculating that the clot was elsewhere in the body and got dislodged. From what medical pundits are saying a year of no contact and blood thinners before you can start evaluating if he can return to football.
That makes sense based on what I have heard. The only tests I have heard about were regarding his heart. If you have an abnormal heart rhythm that can be the cause of clot formation in the heart that can then be pumped out of the heart and often goes to the brain.
I tell people it's like a bowl of jello. If you keep stirring it, it will stay liquid but if it's not moving very much it would become solid.
Depending on the cause blood thinners could be for a year or potentially life long.