It's hard to find a parallel around the league, but yes, having the Vet as the no. 2 on game day makes a lot of sense.
Backups don't get many reps during the week. Should your starter get injured, your backup needs to come in a play basically cold. And run a game plan that may not be suited to him. A rookie without a training camp is setup to fail. Not that he certainly will, but you're not giving him the best chance. Let the vet come in, survive the game, and get out of there.
Now the next week is where you get look critically. If you think your rookie is good, give him all week and hell, name him the starter. Tailor the game plan to his strengths, run those plays in practice, etc.