Azhvars, by themselves, have no where mentioned that they are manifestations of Nitya suris. In fact, they look upon themselves as souls which have been tormented in samsaram from time immemorial. As an example, Nammazhvar says in Thiruvaimozhi (2.6.8): "mARi mARip pala piRappum piRanthu" (I have been born time after time), but how they are special lies in the very next few words of the same pasuram: "aDiyai aDaindhu uLLandhERi IRilinbaththiruveLLam yAn mUzhginan" (I reached your feet and gained clarity of mind, submerged in an endless flood of bliss). It is to be undertood that they are special; they need not attain the Lord's feet after attaining a clear vision of their self, but they have attained His feet and have understood their real nature (i.e. being subservient to Him). Svami Nampillai says : "No one, be it a Nityasuri or a Samsari can be deemed to be equal to (Nam)Azhvar", and the same applies to other Azhvars too. They have been handpicked by The Lord to preach His glories for the upliftment of mankind, by being bestowed with faultless knowledge with which they can comprehend The Lord AS HE IS (Thiruvaimozhi 1.1.1: "mayarvaRa madhinalam aruLinan"). But, for people like us to understand atleast a miniscule of their glories, they have been compared with Nityasuris, since they also possess some of their attributes ("some", because they are far more superior than Nityasuris), like Nammazhvar being a manifestation of Vishvaksena et all. The only concrete evidence of a prophetic incarnation can be seen in Eedu vyakhyanam for Thiruvaimozhi 5.5.1, wherein Nampillai states that it is well-known that Nammazhvar had appeared because of the grace of Thirukurungudi Nambi to Udaiyanangaiyar, Azhvar's blessed mother.