Mat 10:6 But go rather to the lost sheep of the house of Israel.

The word translated as "go" here is poreuomai, which means to "to proceed, "to carry over," or "to convey." "Rather" is mallon, which means "more," "to a greater degree," and "rather."

The word translated as "lost" is apollumi, which means "to destroy' or "to kill." (It was used a few verses ago to describe what happens to old wineskins filled with new wine.")

So Christ isn't just describing the sheep (probaton) of Israel as just astray, but as destroyed.