Absolutely do what you can to work with the moment. But... While the doctors may be giving you bleak prognoses, from experience, they're pretty much winging it when it comes to the brain.
My wife was in a massive car accident. Shattered arm, collapsed lung, multiple breaks to her jaw, cracked eye socket, brain injuries. They induced a coma to keep her alive long enough to get her to a major hospital, called family to her bedside with a prognosis of, "IF she survives the night, it's 50:50 if she'll live." At that point, her brain stem was busy trying to retreat out of the back of her neck.
It was two weeks before they could get any response out of her, another two before she was aware. At that point, they wanted to amputate her arm and told her parents she'd never walk more than a few paces at best, would never look after herself.
Consent was given for the amputation though her mother asked the surgeon to simply do whatever he'd do for his own daughter. He spent eleven hours wiring it together and told her mother he'd most likely be back in to amputate but he'd given it a shot.
Fast forward two years, the arm survived. The girl who'd never walk more than a few paces was out of her wheelchair and starting to try to build a life on her own. In a settlement hearing (she'd bought "unlimited" coverage car insurance for the wreck she was in but there was small print saying they could modify at any time and they swapped it to $100k max five days before the accident) they acknowledged she was lucky to be walking but even the insurance lawyers, whose job was to minimize her injuries, acknowledged she'd probably never be able to return to school. No longer being able to read was a big part of that. And a huge loss to a National Merit scholarship winner, English major and librarian.
About another two years later... I'd taught her how to read again. She'd been living on her own. She returned to school. Started off barely making Cs. GPA went up every semester. She got straight As in her final semester. She now has two degrees, is a certified personal trainer and works in physical therapy. If she doesn't tell people about her injuries, they've absolutely no idea. Not bad for someone who the doctors declared would probably die, would never walk again, never look after herself and never return to school.
To message to take from this is that Traumatic Brain Injuries are absolute bitches but the medical profession has educated guesses about outcomes at best. You read up on neuroplasticity and the like and you realize they're really only just beginning to get an idea of what's possible. There are even stories of key researchers whose family members had strokes, who ignored all of the expert advice and got them back moving again by doing everything "wrong."
So days, weeks, months in... Just because the doctors tell you to prepare for the worst, don't give up. The brain does amazing things, often things they're completely clueless about it being able to do.