Well, well, well, big attention grabber.
This theory is what my bro & I have agreed on, which will make a lot of sense and open more doors to debates & opinions (website NARUTO 101!)

Just before I begin, let me explain that an Anbu member (rank between Chuunin & Jounin) is a shinobi assigned assassination missions, travelling in packs of 3-4 to get rid of dead bodies as if never existed. This is to ensure no thief or villain could get a hold of a clan's secret/ability.

At one point in Kakashi's life, he was an anbu member.
This was probably 4-6 years ago, same time when Sasuke's parents were killed by his bro, Itachi. Kakashi's team were the one's to take care of the dead bodies, so he ordered a medic ninja to give him one of the parent's sharingan eyes. Since Kakashi's not part of the Uchiha family, there were conditions. He could only have 1 eye because of the amount of power it holds and its special abilities from the advanced bloodline.
The operation seemed to have been open-surgery, which is why there's the remainder of a scar on his left eye.