Albeit, the face of Caesar on the coins of the Empire was meant to show the soldiers who their loyalty was to be to.
The coins honor great feats/people, not who the soldiers should worship.
For example, the denarius depicts Caesar, Joshua Graham and Bill Calhoun on it. Are you telling me that the Legion should worship Bill Calhoun or even Joshua Graham? Who Caesar blamed for the defeat on Boulder City and tried to kill by coat him in pitch, set him on fire and throw him into the Grand Canyon by the Praetorian Guard?
The legion soldiers are fanatics, they would not follow a simple soldier that joined the legion ranks just a couple of weeks/months ago.
That's pretty much the same as saying that if a rookie soldier killed Genghis Khan they would become the next Khan, if a rookie soldier killed the roman emperor they would become the next emperor or if a rookie soldier killed a former king in any kingdom they would become the next king.
It doesn't matter what the Courier did while at the service of the Legion, it still doesn't compare to what Legates did during their entire service to the Legion for example.
If Caesar survives, he never bestows any title upon the Courier, that is how much the Legion supports him. Everyone else that distinguished themselves in the Legion gets promoted, but the Courier just gets a picture on a coin.
For the Legion soldiers the Courier is just some person that followed Caesar's orders and got the job done, just like any other at the service of Caesar.
Not to mention if the Courier is a woman, there is no way a female courier would become the Legion leader ever.