This doesn't seem to be the case. For one, my small forwards are equally good (nearly at the same game shape and salary). Yet, this was the most imbalanced one because their minutes were 48 and 0.
What are my options? Should I but better players and overhauling my entire roster?
It's probably more the stamina of your starters is too high. It's unfortunate that the state of the game is such that "stamina too high" is a concept, but if your starters have respectable or higher stamina, it's hard to get them out of the game. If the backup is nearly as good, your best bet is just to alternate which one starts from game to game.