Are you playing on advanced or elite? If not you'll never get injured and need repair.
Ah that explains it. I rarely play after a character hits 50, usually starting over instead. And I generally have been running normal diff when levelling up. Though from what I have been reading it might be better to run higher diffs because of the better drops are in advanced and elite.
Also, it appears (and I was told) that if you keep your ship repaired your injuries will be less.
I.E. If you have no injuries, your first one is likely to be minor. If you have an injury, your likely to get a stronger one the next time you die. The more injuries you have, the higher the chance you will get a crit.
So, keep your ship repaired thru the whole fight, and with luck you will never need to use your critical regerators. Don't know if its true or not, but it certainly appears that way.