Douglas has to have his jaw wired. (Result of an accident? A fight?)
He can deal with the discomfort, he can even more or less cope with the mushy food. What's really killing him is his inability to make snappy retorts. (It both hurts AND comes out incomprehensibly slurred!)

Bonus if Martin at least temporarily enjoys taking advantage of being able to say whatever he likes in the cockpit without fear of Douglas' putdowns.
And then realises that actually he kind of misses it...