It was confirmed, last month, that Mitchell had suffered a brain aneurysm, when she was found unconscious in March.
Crosby said Mitchell, 71, will struggle to recover from the "traumatic brain injury," reported the Huffington Post.
"Nobody found (Mitchell) for a while. She took a terrible hit. To my knowledge she is not speaking yet... She's going to have to struggle back from it the way you struggle back from a traumatic brain injury.
"She's a tough girl, and very smart. So, how much she's going to come back and when, I don't know and I'm not going to guess. I love her. She's probably the best of us - probably the greatest living singer-songwriter. I think we're all holding our breath and thinking a good thought, hoping that it's going to turn out OK," he said.