As always, we must ask why this particular reading from the prophets was chosen to complement a lesson from the Torah portion. The Torah portion begins with the election of Abraham to bring God's message to the world. From the beginning Abraham is told that this is a responsibility that he cannot carry on by himself. It is something that can only be accomplished by an entire people. Thus God promises to Abraham, "I will make of you a great nation. And I will bless you; I will make your name great, and you shall be a blessing. I will bless those who bless you and curse those who curse you; and all the families of the earth shall bless themselves by you." (Gen. 12:2-3)
The haftarah now brings us a message that the prophet Isaiah delivered to the people of Israel as they sat in exile in Babylonia sometime in the 6th century B.C.E. Having been defeated and conquered, the Jewish people had been sent from the land of Israel. Their conquerors surmised that being exiled from their native land, the people would lose their will to go on and assimilate into the general Babylonian population. Yet in exile the Jews remained a distinct people, keeping their belief in God and God's promise to Abraham cited above. Through the words of Isaiah, God wishes to remind the people that though they have left their land, God has not abandoned them. Indeed they will carry on the promise God made to Abraham as "Abraham's nation". This will occur, as Isaiah lets the people know, through a new leader, a "victor roused from the east" (Is. 41.2) who will permit the Jews to return to the land of Israel. Most historians identify this "victor" with the Persian King Cyrus who was willing to let the people go back to the land and rebuild the Temple in Jerusalem.
The concept of the Jewish people continuing the covenant first made with Abraham is found through the connection to the parashah of a number of phrases in the haftarah. The people are referred to as "zera avraham ohavi, the seed of Abraham my beloved" (Is. 41: 8), echoing the promise to Abraham that "I give all the land...to you and your seed (zarecha) forever (Gen. 17:15). So too God says to Abraham,"al tira Avram, Fear not Abram, I am a shield to you", echoed by the prophet who states in God's name, "al tira, Fear not for I am with you, be not frightened for I am your God; I strengthen you and I help you, I uphold you with My victorious right hand." (Is. 41:10)
Finally it is in Lekh L'kha that Abraham and future generations are first commanded to be circumcised (Gen. 17:9-14). There is nothing that would distinguish the Jewish people (physically) as much as circumcision. Perhaps then the haftarah is also connected to the Torah portion through what continued to be both a symbol of their covenant with God, and at the same time maintained their distinctiveness as a people -- brit milah (circumcision) -- until the exile was over. As Jews read both the Torah and haftarah they were comforted that the covenant first mentioned in Genesis continued to apply to Jews where ever they lived, whether in Babylonia or throughout the diaspora, symbolized through their own brit milah. Isaiah's message thus proved to be a comfort not only to those who first heard it, but also throughout the generations, whenever it was read.