Under what circumstances were Union soldiers injured in battle sent home? This is what some call the "million-dollar wound". Were wounded soldiers sent to a hospital to recover and then return to their regiment?
It probably was a judgement call based on each individual case. Looking at what happened with the soldiers from my town in Maine, not many of those who were discharged for disability re-enlisted. In several cases, they were discharged because they were dying and were sent home to die there; these men had to be able to travel. Then there were cases like Joshua Chamberlain, also expected to die, but clearly unable to travel. In the end, I'd guess that the hospitals took in the most hopeful cases--and the least hopeful.