Textual analysis: This verse is somewhat difficult to understand clearly because it so obviously shifts between a plural and a singular subject in the first and second sentences. The relationship between these is best seen as a rhetorical question to the multitude assembled, with the second sentence referring to the answer. The Lord address "All ye..." He also then refers to "them," apparently indicating a different referent. Who is represented by "them?" It appears probably that the Lord is speaking to current Israel, and that the "them" refers to past Israel, as the context of the passage is prophecy that has apparently been given.
The Lord next indicates: "the Lord hath loved him; yea, and he will fulfil his word which he hath declared by them..." Once again there is that problematic compounding of the singular and the plural. Perhaps there is some facility in Hebrew which might explain this, but I am at a loss for the consistent conflation of the two in this verse. Accepting the English translation, we would need to see the "him" as a generic reference of the singular standing for the plural, and the passage referring to multiple past prophets rather than a single one.
In Avraham Gileadi's translation, he removes this difficulty:
The plural/singular problems are removed in this translation, with the individual as the primary referent prophet rather than multiples.
In either case, it is clear that it will be the Lord who will be the agent of the fulfillment of the prophecies.
Concerning this passage, Victor L. Ludlow notes:
"To fulfill his purposes, the Lord will bring forth a servant who will foretell the future, fulfill the Lord's word, wield power over Babylon, and ultimately succeed in his foreordained mission. Although Isaiah or Cyrus could possibly fit the description of this servant, the Lord Jesus Christ best exemplifies these qualities."
Rather than see a single fulfillment of this statement, I think it more likely that all of the potential players serve as a partial fulfillment. Prophecy is by its nature (and probably its purpose) a fairly vague thing, without such uniquely identifying characteristics that the fulfillment of prophecy is known before hand with certainty. The fulfillment of prophecy is known after the fact of the fulfillment, and in the Lord's conservation, a single prophecy may have multiple fulfillments. We have, for instance, the millenial prophecies of Matthew that found a fulfillment in the destruction of Jerusalem by the Romans, and are yet seen as descriptors of the final scenes before Christ's triumphant return. Neither is incorrect.