Ken makes a sound of unmistakable hurt, and he makes a painful, abortive effort to roll over and face away from Chikusa. It'll actually succeed if Chikusa lets him do so; the 'abortive' part comes in with the fact that Chikusa will undoubtedly stop him before he can complete such an ill-advised move. He can't begin to vocalize the tangled mess of emotions Chikusa's reassurances have given him, but none of them are 'reassured'. The only part that sounds even vaguely promising is the possibility - not even a certainty - that they might find something that can heal him faster.

The rest...there's nothing there to make him feel better. There's no promise he won't be agonizingly bored, that he won't be left alone and helpless while he heals. And there's the quiet implication that Chikusa and Mukuro can get by fine without him, which begs the question of why they even need him. A question that they'll have months to mull over.

He can't articulate that mess of pain, so all he can do is squeeze his eyes shut and curl up in pain that ignores the physical agony he's in completely, in favor of the emotional hurt that there's no box to heal.