The defense was in a loose "prevent" in the third quarter after taking the 22-3 lead. It's been a Frank characteristic to pull the plug early when building a nice lead. It hasn't happened much this season, but he does it out of habit, I guess. Frank gets conservative quite often.
And when you get right down to it, the kicker cost 'em the game, missing an extra point after the 2nd score and of course missing the las FG try. If he was truly injured, you are correct that they shouldn't have been playing for a FG to win the game.
But that's my opinion, yours might vary.