A claim that if there is a god, then "intelligent design" is likely is based upon several undemonstrated propositions. First, that if there were a god, it would be a creator god--and even a cursory review of human concepts of deities shows that there is no universal assumption that any particular god or set of gods are creators. Second, it is based upon an assumption that if there were a god, that god would be omniscient and omnipotent. It is rather obvious that there is an assumption that if there were a god, that god would be the same as the god of the Jews and Christians. Even that god, as described in scripture, is clearly not omniscient and omnipotent--either that, or that god is almost inconceivably cruel and sadistic. The entire proposition is a glaring example of the
fallacy of the excluded middle. This is an assumptive fallacy, and at the linked page, this is the discussion:
Where people have a high need for certainty and control, extreme views provide what may seem to be a defensible position, as you only need to look in one direction for the 'enemy'. Dividing the world into good and bad, right and wrong also plays to certainty needs, as your friends and enemies can now be clearly identified. Assuming that any god would be a creator god, and would be omniscient and omnipotent is the source of the fallacious argument that if there were a god, intelligent design would automatically be possible. Once again, far too many people assume that if they can conceive of something, it is possible.