Depression and/or Bi-polar (manic depression) can take an awful long time to overcome to any meaningful level so that you can function as a 'normal' person. Outwardly you look like any other tom, dick or harriet & seem completely okay but inside is a different matter altogether and your ability to cope with every day stuff (never mind been at work) goes right out the window.
There's also PTSD which whilst many associate it with war and major catastrophe's (The recent Norwegian incident springs to mind) episodes/events in people's lives can have similar deep rooted effects that can last a lifetime.
Personally I've experienced it on both sides of the coin and I hope that those around Gleeson can get him the help he needs. However mental illness is far more damaging & far reaching than most other physical injuries so there is a very very high chance he won't return to playing rugby at the top level if at all.
If he thinks he may well not come back he should do the honorouble thing and quit without taking next years pay or come to an amicable agreement with the club to help his near future.