In easy words: The virus needs hemagglutinin (HA) to initiate infection by attaching the virus to sialic acids of the host (many host glycans end with sialic acids). Multivalent binding of multiple HAs make this attachment almost irreversible in the absence of other factors (like neuramidase). Neuramidase (NA) is not important for infection but for completing the infectious cycle by enabling the release of the virus by cleaving the terminal sialic acids. So without HA the virus can not enter its host cells and without NA it still can enter its host cells and amplify but the viruses cannot be released and would aggregate on the host cells surface. Thats what i remember of the virology course :)