Sri Swami Vishwananda said in his speech on Hanuman Jayanti 2010:
"Mahavishnu decided to incarnate as Rama and after that, He incarnated alone. So all the Deities, all the Devas incarnated.
At the same time He was asking Shiva: 'You are the Lord of the Universe, you are realised. You are one with God all the time, but tell me one thing:
On whom are you meditating, like that? You always sit in meditation, but for sure you are meditating on somebody.'
Shiva replied: 'Yes, all my thinking and all my mind, all my concentration, my meditation is only on God, Mahavishnu. So whenever I meditate it’s on Mahavishnu, that I’m meditating and my Love for Him doesn’t have any limit. So now that He is going to be incarnated on earth as Rama, I’m thinking also to be incarnated.'
So Hanuman is a manifestation of Shiva. He is a pure incarnation of Shiva actually. He is not just part of Shiva, but a full manifestation of Shiva.
Even if Shiva has space in Shiva Loka, in His body, He is a full manifestation of Hanuman. That’s why it’s said that if you can not pray to any Gods, pray only to Hanuman - you will have everything. And when you chant the Hanuman Chalisa, this part of the Hanuman Chalisa that says: 'I don’t pray to anybody, but only to you.'
'Aura Devata Chitta na dharayi,
Hanumata se yi sarva sukha karayi'.
It means: even if you don’t pray to any Deity, but only to Hanuman, you will receive the blessing of all the Deities."