It is a natural instinct to affirm Tawheed al-Uluhiyah which is proved by Tawheed ar-Rububiyah because every person clings to the origin of his creation and the source of his benefit and harm. He then turns to the means which bring him closer to Him, please Him and strengthen the bond between them. Thus, Tawheed ar-Rububiyah is a door to Tawheed al-Uluhiyah and thus Allah used it as a proof against the Mushrikeen (those who associate a partner with Allah), Allah says in the Qur’an
"Say, 'Whose is the earth and whosoever is therein? If you know!' They will say, 'It is Allah's!' Say, 'Will you not then remember?'
Say, 'Who is (the) Lord of the seven heavens, and (the) Lord of the Great Throne?' They will say, 'Allah.' Say, 'Will you not then fear Allah.'
Say, 'In Whose Hand is the sovereignty of everything (i.e. treasures of each and everything)? And He protects (all), while against Whom there is no protector, if you know.' They will say, '(All that belongs) to Allah.' Say, 'How then are you deceived and turn away from the truth?" [Soorah al-Mu'minoon (23): 84]
"Such is Allah, your Lord! None deserves worship but He, the Creator of all things. So worship Him (Alone)." [Soorah al-An'aam (6): 102]
So, Allah used His Oneness in Lordship as a proof to establish that He alone deserves to be worshiped.
The slave cannot be a Muwahhid (one who believes in Tawheed in the correct manner) by affirming Tawheed ar-Rububiyah alone -until he declares Tawheed al-Uluhiyah and establishes it. The Mushrikeen used to proclaim Allah's Oneness in Rububiyah but it did not qualify them to enter Islam. The Messenger of Allah fought them while they were declaring that Allah is the Creator and Provider, the One, Who gives life and death as Allah says, "If you ask them who created them, they will surely say, 'Allah.''[Soorah Zukhruf (43): 87]
"Indeed if you ask them, 'Who has created the heavens and the earth?' They will surely say, 'The All-Mighty, the All-Knower created them." [Soorah az-Zukhruf (43): 9]
"Say, 'Who provides for you from the sky and from the earth? Or who owns hearing and sight? And who brings out the living from the dead and brings out the dead from the living? And who disposes the affairs?' They will say, 'Allah.'" [Soorah Yunus (10): 31]
Plentiful similar verses can be found in the Qur'an. He, who claims that Tawheed is to affirm the Existence of Allah or declare that Allah is the Creator and the One, Who disposes the affairs of the Universe, and restricts himself to this definition -then he does not know the reality of Tawheed to which the Messenger called. Because he has halted at the affair, which necessitates (i.e., Tawheed ar-Rubibiyah) and he has abandoned what it necessitates (i.e., Tawheed al-Uluhiyah) or he has stopped at the proof and abandoned what it proves." [end quote from Aqeedah at-Tawheed]