Danzo would be an idiot not to put an eyeball in the socket if he was missing an eye, the technology existed for hundreds of years and even genin medical shinobi could do the procedure. He could have taken any eyeball of anybody who died over the 4 or more years that he had the bandages before the uchiha incident, i doubt he would just sit there with no eyeball and bandages on his arm that works perfectly fine. Even if his arm was burned it woulda healed years before this flashback.