That is not necessary, but recommended. I mean you will get replacements faster for being passive, or in a depot, or in a city. What really matters to get replacements is that the units that need them do not move during that turn.
Regards!
[color="Navy"][font="Georgia"]"Mi grandeza no reside en no haber caído nunca, sino en haberme levantado siempre". Napoleón Bonaparte.[/font][/color]
If you are playing with hardened attrition on they have to be in a region with a depot to receive whole elements as replacements, but existing elements will gain strength without a depot.