Whenever, therefore, respecting the Godhead itself, containing both deity and dominion, the term God is distinctively used, it is applied to the Father, and Lord to the Son.
Ib.p.

281.
But, farther, it is objected that Christ cannot be God, since God calls him 'his servant' more than once, particularly 'Isaiah' xlii.

1.
The Prophets often speak of the anti-type, or person typified, in language appropriate to, and suggested by, the type itself.

So, perhaps, in this passage, if, as I suppose, Hezekiah was the type immediately present to Isaiah's imagination.

However, Skelton's answer is quite sufficient.
Ib.p.