After being told on the headset (possibly with one engine already running) that the tug has pushed the aircraft onto the grass - and can't pull it forward - the responsibility of the Captain is to secure the aircraft, deal with the passengers and hand over to FOPS. It's certainly not his prerogative to ascertain whether the aircraft could/should be moved under its own power. Way above his pay scale and fraught with danger.
Having done all of the above, gather the crew and get a coffee, asking Crew Control what they plan to do with you all after such a traumatic incident (couple of de-stressing days off would be nice).
