I think fifteen years ago Bradley would be more than fine, but the way the game is officiated nowadays makes it very hard on centers. Any real physicality is not allowed, thus, in order to protect the paint, centers must have an unlikely combination of lenght and athleticism. Most centers today, even the league starters, are simply not up to this task. And if the starters are not up to the task, what one can say about the back-ups? I really cannot think of any current back-up in the league that could sub in for Gobert and we would go "Woohoo! No problem!"
Some posters here are clearly Favors' widows, but the fact is that he is not available. Besides, he is also payed as a starter, not as a back-up.