I referred to these as the “dogs that didn't bark,” in reference to the Sherlock Holmes' novel Silver Blaze (Tony Lane informed that Msgr. Ronald Knox called this form of argument “Sherlockismus”). If the early Church really was Protestant in their Eucharistic theology, we should find them teaching obviously-Protestant things about the Eucharist, or at least objecting when prominent Church members proclaim obviously-Catholic things.
A Protestant reader turned this question on its head, and asked: if the early Church took Jesus' words here literally, where was the Jewish outcry? After all, it was contrary to the Law to drink blood, and the whole practice seems barbaric from the outside. That's a great question. And fortunately, Catholics can point to plenty of places where we do see this outcry.
A few things to remember. The first is that there's no question that the early Christians didn't keep kosher, and that this was a major cause of controversy. We see the debates over this (called “the Judaizer controversy”) in places like Acts 10-11, Acts 15, Paul's writings, etc.
Second, it's not clear that the Eucharist is a violation of the Law. Certainly, it's not against the Spirit of the Law. The Eucharist isn't consumption of Blood in the normal sense. Otherwise, we'd be cannibals. Although it's literally Jesus' Body and Blood, we aren't destroying Jesus in the process (more on this in the next section). So it's no more against Jewish kosher laws than it is against the Christian prohibition against cannibalism. For example, Origen argued that the Eucharist didn't count as breaking a fast, because it's not ordinarily eating and drinking. Similar logic would seem to apply here. Besides that, non-Christian Jews would have believed the Eucharist to be just bread and wine. So they were likely less scandalized by it then they were by the Christian practice of eating pork, for example.
Third, we actually see this outrage in John 6. In particular, read John 6:52-58:
The Jews then disputed among themselves, saying, “How can this man give us his flesh to eat?”
So Jesus said to them, “Truly, truly, I say to you, unless you eat the flesh of the Son of man and drink his blood, you have no life in you; he who eats my flesh and drinks my blood has eternal life, and I will raise him up at the last day. For my flesh is food indeed, and my blood is drink indeed. He who eats my flesh and drinks my blood abides in me, and I in him. As the living Father sent me, and I live because of the Father, so he who eats me will live because of me. This is the bread which came down from heaven, not such as the fathers ate and died; he who eats this bread will live for ever.”
Note that Jesus doesn't correct the Jewish objection as a misunderstanding. He reinforces it, instead. After this, many of His disciples denounce it as a “hard saying,” and stop following Him (John 6:60, John 6:66). In other words, this is exactly the sort of outrage we'd expect to find... and we find it from the very beginning.
This dog also barked for the Romans, who accused the Christians of being cannibals, for covering baby Jesus with grain, then tearing His Flesh apart and drinking His Blood. See, for example, the reference in Minucius Felix's Octavius (which was written sometime between 150-270 A.D.):
Now the story about the initiation of young novices is as much to be detested as it is well known. An infant covered over with meal, that it may deceive the unwary, is placed before him who is to be stained with their rites: this infant is slain by the young pupil, who has been urged on as if to harmless blows on the surface of the meal, with dark and secret wounds. Thirstily - O horror! they lick up its blood; eagerly they divide its limbs. By this victim they are pledged together; with this consciousness of wickedness they are covenanted to mutual silence.The Roman critic appears to have gotten the details of the Nativity and the Eucharist all mixed together. Which is itself significant. The Nativity story involves a journey to Bethlehem (which means “House of Bread” in Hebrew, and “House of Meat” in Arabic), and placing Jesus in a manger, that is, a food trough. Jesus' Flesh is the Bread upon which Christians feed. So the Romans were inadvertently right in seeing a connection to the two, even if they screwed the details up badly.
An English Lutheran put it simply: “If what you believe and teach concerning the Supper couldn’t be misinterpreted by some people as sounding like cannibalism, then your understanding and/or teaching of the Supper is deficient.” The early Christians believed something about the Eucharist that sounded like cannibalism to outsiders. If we don't believe that today, we've lost their faith. And when Jesus' Jewish critics accused Him of teaching that He was going to give us His Flesh to eat, He didn't deny it, but reinforced their point. The Catholic Church would respond the same way today. Would you?