When a defeated adventurer regains HP so that it is not defeated, it is deployed to the same hex it occupied previously.
Note, in some very rare cases, adventurers may be defeated by a game effect that defeats units on specific hexes so that other units can go there. A unit cannot be deployed to an occupied hex, so a unit defeated in this way cannot be resurrected with Master Ritualist, since it cannot be deployed to the hex it needs to be deployed to.