The manager was stacking my transaction before I set the animation, so it stacks the transaction without animations (sad but true), and that occurs even if I commit the transaction after the setCustomAnimations().
The solution is to set the animations first:
FragmentTransaction transaction = manager.beginTransaction();
transaction.setCustomAnimations(R.anim.slide_in_left, R.anim.slide_out_right);
transaction.replace(R.id.content, myFragment);
transaction.commit();