I think that the allusion to Yeats helps to bring out the chaotic nature of the world in which Okonkwo lives. The idea of "the center cannot hold" is reflective of the world to which Okonkwo returns only to find that colonialism has transformed the traditional values of his tribe. The nature of "anarchy loosed upon the world" is an idea that is internal to Okonkwo's sense of perception in that there is little in way of what had previously defined structure and understanding.The presence of the colonial forces in the village eradicates the tradition and understandings that helped to provide meaning for Okonkwo. The changing world is one that throws his sense of order and sequence awry. Yeats' contention in the first stanza of the poem is the depiction of a world in which there is no structure nor semblance of order. This would have to be where Okonkwo lies in his perception of reality. In Achebe using Yeats' poem, the basic timbre of a world where loss of structure and order is evident is echoed in Okonkwo. In this, more about the poem is made relevant to the cultural setting in which Achebe writes.