I have two NT player in my team and i got 140k in merch three weeks ago. But the number drops to 80k in the next week since i bought 2 new centers for my team in that said week. I am worried about last week's update since i lost both games but my merch came back again to 140k.
So if you want a good merch income, have a player who is on NT esp. local players. Also try to be an all-Local player team and I assure you your merch will rise. About the rookies, it's better to train and make your rookie as your key player.