The matter must be adequate to the form. Inadequate matter does not mean that the form is not possessed, but that the full exercise of its powers is rendered impossible.
The form is possessed if the subject in question is a homo sapiens. That formally entails rationality. What actually obtains may be delimited by material impediments.
It is similar to when a man has an amputated leg. Homo sapiens is a bipedal animal. The amputee is still formally a biped; if he were not, either homo sapiens has changed or he is no longer a homo sapiens. Neither of these is the case; he would be a biped if his matter were fully adequate to his form, but it is not.
The same applies to head trauma, such as the famous case of Phineas Gage who had the iron tamping bar blown straight through his cranium (!) in a demolition accident. He experienced certain cognitive disorders as a result. The essence of reason did not change in the man, nor did he lose his essence as a man. But the form homo sapiens is a composite form, which requires adequate instantiation in matter.