Whatever the actual reason for choosing the liver was, we can be certain it wasn't its regenerative property, since such proprerty was unknown to the Greeks at the time.
Just went and checked, evidently he never said anything about it.
old American translation of Galen and Hippocrates for anyone interested, as it can be difficult to find otherwise
https://oll.libertyfund.org/titles/coxe-the-writings-of-hipp...