Yes it came from a BB, though he didn't express it in the same terms.
You start out with the original injury, which is a value between 0 and 10.
By mapping, I mean converting values in the original range of 0 to 10 to a new range. So if you had a level 8 doctor, you would map the range 0 to 10 to 0 to 9. Which is the same as multiplying by 9/10.
It may be easier to reverse the process.
If you have a level 10 doctor, injuries that were originally 0 to 5 would only last for the rest of the game. This is more than 1/2 of all injuries.
If you have a level 1 doctor, injuries that were originally 0 to 0.9 would only last for the rest of the game. So a much larger share would happen after the game.
If you had a level 6 doctor, injuries that were originally 0 to 3.75 would only last for the rest of the game.
If you had a level 10 doctor, injuries that were originally in the range 9 to 10 would be +4 to +5 after the game.
With a level 1 doctor, injuries that were originally in the range 8.2 to 10 would be +4 to +5 after the game, so almost twice as many would be of the longest duration.
With a level 6 doctor, injuries that were originally 8.75 to 10 would be +4 to +5 after the game.
So you can still have +5 injuries with any doctor. These would be rare to start with, and top doctor would make them even rarer.