Why were the disciples surprised? They were surprised because they had a veil over their eyes that was suddenly and unpleasantly ripped away. Matthew 26 is not telling us that the Old Testament is inherently Christless. It is telling us that the disciples did not understand.
There is no doubt that the disciples were surprised to learn that Jesus’ death and resurrection was the point of the Old Testament. In Matthew 26:54-56, it was immediately after Jesus says THIS (His arrest and death) was to fulfill Scriptures that the disciples left Him. Let no one therefore think the interpretation of the Old Testament to be a matter of indifference.
It is commonly debated today, however, why the disciples were surprised. If one compares this passage in Matthew with Luke 24, for example, we come across a bit of a puzzle. How can Jesus reproach the two disciples for being slow to believe all that the prophets had spoken? And in 1 Peter, why did the Old Testament writers search so eagerly in their own writings? It must be because they knew that there was something more in what they wrote than what they themselves had thought. They understood that they had written the Word of God and that God had further things to say than they, the authors, had intended.