I've been looking at this quite extensively, in particular using Keil and Delitschz's commentary. In their notes on Isaiah 42:1, they say,
Now the "servant of Jehovah" is always Israel. But since Israel might be regarded either according to the character of the overwhelming majority of its members (the mass), who had forgotten their calling, or according to the character of those living members who had remained true to their calling, and constituted the kernel, or as concentrated in that one Person who is the essence of Israel in the fullest truth and highest potency, statements of the most opposite kind could be made with respect to this one homonymous subject. In Isaiah 41:8. the "servant of Jehovah" is caressed and comforted, inasmuch as there the true Israel, which deserved and needed consolation, is addressed, without regard to the mass who had forgotten their calling. In Isaiah 42:1. that One person is referred to, who is, as it were, the centre of this inner circle of Israel, and the head upon the body of Israel. And in the passage before us, the idea is carried from this its highest point back again to its lowest basis; and the servant of Jehovah is blamed and reproved for the harsh contrast between its actual conduct and its divine calling, between the reality and the idea. As we proceed, we shall meet again with the "servant of Jehovah" in the same systole and diastole. The expression covers two concentric circles, and their one centre. The inner circle of the "Israel according to the Spirit" forms the connecting link between Israel in its widest sense, and Israel in a personal sense. Here indeed Israel is severely blamed as incapable, and unworthy of fulfilling its sacred calling; but the expression "whom I send" nevertheless affirms that it will fulfil it - namely, in the person of the servant of Jehovah, and in all those members of the "servant of Jehovah" in a national sense, who long for deliverance from the ban and bonds of the present state of punishment (see Isaiah 29:18). For it is really the mission of Israel to be the medium of salvation and blessing to the nations; and this is fulfilled by the servant of Jehovah, who proceeds from Israel, and takes his place at the head of Israel. And as the history of the fulfilment shows, when the foundation for the accomplishment of this mission had been laid by the servant of Jehovah in person, it was carried on by the servant of Jehovah in a national sense; for the Lord became "a covenant of the people" through His own preaching and that of His apostles. But "a light of the Gentiles" He became purely and simply through the apostles, who represented the true and believing Israel.
http://biblehub.com/commentaries/kad/isaiah/42.htmSo to sum this up, the term, the Servant of Jehovah, can be used by the author to denote either
1. The whole nation of Israel, which had forgotten its calling
2. The faithful members of the nation, who'd remained true to their calling
3. The one individual representative of Israel who would fulfill Israel's mission to the world, and under whom Israel would continue in that mission.
The quote below expands on point 3:
In the first part of the book of Isaiah, the anticipation of a new king who will help God’s people respond faithfully to Him gradually comes to the foreground (9:2-7, 11:1-16). Israel’s past kings had miserably failed. So the people longed for a new intervention by God. God had raised David to establish the nation. Now the people yearned for a righteous and just king like David (11:3-5) who would teach them how to be God’s people (note Psalm 72).
http://www.crivoice.org/isa53.htmlClearly the coming king mentioned in the passages quoted here is in the mind of Isaiah as he writes the passages known as the 'Servant Songs'.
This may also be why Matthew begins with a genealogy that shows Jesus to be an Israelite by descent, qualifying him for the role as 'the Servant who is the true Israel'.
