Marshall isn't leaving the starting rotation until he is playing in the NFL. The couches have started getting him more involved in the offense in the last three weeks, and he has looked great. The one thing that really makes me a believer is his ability to run between the tackles. If the H-Back is a threat to run inside, outside, or catch the à la Percy Harvin, Meyer's spread offense can be run to its full potential. While he doesn't have Dontre's raw speed, Marshall is a more violent runner (With some throwing ability to boot) who defensive coordinators will have no idea how to deal with (When Wilson is lined up at running back it seems safe to say the ball is going to the outside of the field).
A player should never lose his job because of an injury, but Dontre is going is still going to find plenty of touches in an offense where positions and titles are basically meaningless and he can still be a great player. But I'm a big believer in Marshall being the non-quarterback player our offense is going to revolve around for another year or two.