Matthew’s quotation of Isaiah 53:4 in 8:17 is not from the LXX. It is either Matthew’s own translation, or a quotation from another Greek translation that is no longer extant.

Matthew’s quotation is closer to the Hebrew than the LXX, but according to Davies and Allison, Matthew has shifted the sense of the Hebrew text: “In Isaiah the servant suffers vicariously, carrying informities in himself; in the Gospel he heals the sick by taking away their diseases. In the OT the distress seems to be mental or spiritual; in Matthew physical illnesses are the subject. So a text about vicarious suffering has become a text about healing, and two different pictures are involved.” They suggest that Matthew sees healing as a “type of Jesus’ redemptive suffering,” or that “the association between sin and the distasteful reality of disease was so intimate . . . that the healing of sickness could be conceived of as a taking away of sins.”

But there doesn’t seem to be any conflict to resolve here. Matthew shows us Jesus healing, often by touching (8:1, 15). At the end of the first triad of healings, Matthew unveils the reality of the healing by quoting from Isaiah. Isaiah tells us that the Servant - whom Matthew has already identified as Jesus (3:17) - takes our weakness and experiences our sicknesses. Matthew says that this is what Jesus is doing. Like his other fulfillment formulae, 8:17 is not simply a straightforward this-is-that equivalence; the quotation from Isaiah is a theologically weighty explanation of what is happening when Jesus heals.