There was a lot to do, but steady and stubborn effort can get her through it; as for Optimus, he seems minimally responsive at best. It's not the first time he's fallen in to an exhausted recharge during a medic's mercies (as Ratchet can vouch), but it's been a long time since he had any sort of such luxury.

It means he feels, at least, for the moment, safe.

Which says quite a lot.