God doesn't burden anyone beyond their ability, so His commanding them with circumcision must have been in their ability. Just as He gave them an intellect and the ability to build, He also provided what was necessary for them to fulfill their tasks.
Perhaps the discrepancy is in your belief that 'primitive' humans only knew opium and cauterization as medicine for circumcision. Humans have been using herbal medicine in the form of leaves, oils and ointments for centuries. It's completely illogical to think that 'primitive' humans would not have had the same access to this, apart from cauterization. The very fact that they used herbal medicine and cauterization suggests that they did know about germs and infection. Not to mention that there are new methods of circumcision that use a string and require no cutting at all.
As for anesthetic, I have two sons who were circumcised as infants - one with the string and one I cut with the assistance of a Jewish mohel - and neither one of them required anesthesia and were fine within a matter of minutes. Perhaps 'primitive' humans and babies are a little tougher than some men these days.