There were, then, among the Jews certain men who were prophets of God, through whom the prophetic Spirit published beforehand things that were to come to pass, ere ever they happened. ... And He was predicted before He appeared, first 5000 years before, and again 3000, then 2000, then 1000, and yet again 800; for in the succession of generations prophets after prophets arose. First Apology, Chapter XXXIFirst Apology, Chapter XXXI
So, which OT canon did the early church recognize? The answer is it recognized what the Protestants recognize now and do so for the same reason.