This is a good question, but I think that some level of clarification might be needed. I am not sure Yeats' poem is meant to be read in a Postcolonial light. I think that he is writing in a more Modernist setting, concerned with the state of Europe at the time and I am not sure he is reaching into a Postcolonial frame of reference, something that was far off from Yeats' reach in his writing. Achebe's work is directly Postcolonial, a statement about the relationship between indigenous people and the condition of colonization that is a part of their being. It might be important to lay this out at the start. I still think that the thematic link between the two can be forged, but it has to be stressed that one might be reaching in solidifying the Postcolonial links in Yeats' work.
One distinct similarity that is in both the poem and the work, connecting to a Postcolonial reality is how there is no existing structure in the world after Colonialism. Yeats' poem speaks to the death of this order. Achebe's work speaks to the struggle that Okonkwo faces in a setting where identity and a sense of meaning are both noticeably absent. Another link that can be extended from this is that when one tends to view meaning as being provided by external structures, there is an existential crisis that happens when this structure disappears. For Achebe's work, the struggle to understand identity when the Ibo cultural norms are dissipated and a colonial setting is in its place can be matched up with Yeats' vision of a world in which external structures have failed to provide this meaning to individuals who came to depend on it. The Postcolonial strain in both is how meaning will be determined in a world that has fundamentally changed and shifted in to a more unknown sense of being. In this, one can see how Achebe's work, Yeats' poem, and Postcolonial reality can come together.