From the context of the verse you quoted, the Messenger had 'wives' before the Zaid situation arose.
33:28 O Prophet! Tell your wives, “If you desire the life of this world and its glitter – well, then, I shall provide you to your contentment and let you go in a becoming manner.” [The Prophet had chosen to keep a standard of living of the poorest of Madinah, and the mothers of believers were seeing other women living in relative affluence]
33:29 “But if you desire God and His Messenger, and the Home of the Hereafter, then certainly, God has prepared for those among you who benefit humanity, an immense reward.”
33:30 O Wives of the Prophet! (You will be seen as role models for other women, therefore), if any of you shows unseemly conduct, double would be her suffering, for, that is easy for God.
33:31 And whoever of you is devoted in the service of God and His Messenger, and helps improve the society, We will give her reward twice over. We have readied for her a most excellent sustenance.
33:32 O Wives of the Prophet! You are not like other women. If you seek to live upright, then, be not overly soft in speech, lest he in whose heart is a disease aspire (to you). But speak in customary kindness.
33:33 And when you abide in your homes, do so in quiet dignity, and do not make a dazzling display like that of the former Times of Ignorance. Help establish the Divine System and the Just Economic Order, and obey God and His Messenger. God wishes to keep away vice from you, O Members of the Household! And to purify you to utmost purity (of character).
33:34 And bear in mind all that is recited in your homes of God’s verses and wisdom, for, God is Sublime, Aware.
33:35 Surely, for Muslim men and Muslim women, believing men and believing women, devoted men and devoted women, truthful men and truthful women, steadfast men and steadfast women, humble men and humble women, charitable men and charitable women, abstinent men and abstinent women (abstaining from all vices), chaste men and chaste women, men who remain mindful of God and women who remain mindful (of God), for them, God has readied forgiveness and an immense reward. [3:194, 4:124. Maghfirah = Forgiveness = Protection against detriment = Erasing the imprints of faults = Absolving imperfections]
33:36 It is not fitting for a believing man or a believing woman, when a matter has been decided by God and His Messenger, to claim freedom of choice concerning themselves. And whoever disobeys God and His Messenger, he has obviously gone astray.
33:37 (In personal matters you may respectfully disagree with Muhammad in the capacity of a human being 3:79. As an example) Recall that you did tell the one to whom God had blessed and you had blessed, “Hold on to your wife and be mindful of God. You seek to conceal within yourself what God intends to reveal. You fear people whereas it is more appropriate to fear God.” But when Zaid had concluded the marriage and divorced her, We gave her to you in marriage, so that henceforth there may be no blame on the believers in respect of marrying the spouses of their adopted children when they have come to the dissolution of their union. The directive of God is always fulfilled.
33:38 There could be no difficulty to any Prophet in what God has assigned to him as a duty. That was God’s law for those who passed on before. And the commandment of God is a determined decree.
[God helps His Messengers and they consider none of their duties difficult in spite of all the strife involved. Sunnatillah = Laws of God. In the World of Command He makes laws as He wills. Then He implements these laws in the World of Creation (the Universe). And then He neither changes them, nor makes any exceptions. 7:54, 17:77, 33:38, 33:62, 35:43, 40:85, 48:23]