Uhh, Utah wasn't supposed to be good at all this season, not sure how they're "underachievers." You don't let Al Jefferson and Paul Millsap walk and take on $24+ million in a salary dump of bad/mediocre players (along with two first-rounders!) with the intent of being good. Are the Celtics underachieving?
They have the 29th ranked offense and a bunch of young, inexperienced players - that's why they're terrible. Not because Favors is a bad defender; it's entirely the opposite with him. He earned that contract purely because of his defense; the question was with his offense, which Utah believes will eventually come (as it is starting to do).