How so? I'm not questioning your basis, I'm curious what you mean. I'm aware of Galen, and his (for then) breakthroughs in treatments. But how did that hold back further advance? If you're saying that his stature and credibility were such that later research was stifled if it contradicted Galen... well, that's probably true, but it's not the fault of Galen, it's the nature of the scientific community. Anybody that questions the conventional wisdom in science is suppressed until they can no longer be ignored, if they're right.
I'm not even admitting that it necessarily a bad thing. It's just the way scientific thought works.
Isn't that sortof like saying that Copernicus delayed the advent of more modern astronomy? That's not a fair accusation. It's just the natural story of the progress of science.