Naturally, someone like Lyu Bu, whose immense power is completely beyond comparison with any deity, in the minds of the Kushan people who have partially assimilated Hinduism and then been counter-assimilated, must have a corresponding match.
And this battle power, so strong it's against the heavens, should naturally be equated with the strongest, most ancient deities. So in their eyes, Lyu Bu is the avatar of Mahesvara.
After all, only this deity, seemingly part of Buddhism's Twenty-Four Heavens, fits their view of the strongest god, as in Hinduism he is the ultimate deity, known as Mahesvara, the Creator God of destruction and genesis.
In any case, since we are all gods' harbingers in the Mortal World, avatars of gods and buddhas, then Lyu Bu must be one too. And given his dramatic entrance and group battles, only the strongest, officially responsible for destruction and genesis, the Main God fits his situation.
