It pulls at something inside to see his former professor like this, but it fuels his hope as well. Even if Callaghan won't voice it, Tadashi can see the regret. Hear it. It's practically rolling off off of the older man.

Remember that hug Tadashi tried for the night Callaghan saved him? Well he's trying for it again, and this time without microbots holding his feet in place.

"I know."

No one could've known how things would end up.