The Bhagavad Gita (VIII,18) says, 'from the unmanifested, all the manifested proceeds at the coming of the day; at the coming of night, they dissolve in that, which is called unmanifested.' It is this 'Unmanifested' that is called Avyaktha Roopa. Baba is the Sovereign of Creation, existence and dissolution. He says that "wherever you find the three activities or Sakthis of Srishti, Sthithi, and Laya, there you can recognize God". Dissolution is into the unmanifested, existence is the intermediate state of manifestation. So, Baba is Avyaktha Roopa, as well as Vyaktha roopa. As Vyaktha (Manifested) He is Sathya Sai Baba, as unmanifested He is Niraakara, the formless Datta-Sakthi, Narayana-Siva-Brahma Sakthi.
In the Gita He says "Though I am unborn, and eternally unaffected, I happen with name and form, utilising the elements of Prakriti, nature"; When He uses Akash as the base, the name and form are Vishnu, the Omnipresent: with Vayu as the base, He is Surya effulgence, light, wisdom; Agni as the base, He is Maheswari, the mother, the eternal feminine; with Jala as the base, He is Vinayaka, or Ganesa, Vighneswara, the guardian who saves from disaster and downfall; with Prithvi as the base, He is Siva, the Vibhuthi-Bhushana, the ever-merciful.
Unmanifested, He is all these Gods; (Vishnu, Surya, Durga, Ganesa and Siva). He is Sarvadevatha. All Gods in one.