If he indeed had no lapse of service, nor anything notated in his service record cards, it would normally be assumed that the reported injury was quite minor. Treated and returned to duty. They didnt do the steel plate in the head thing in the 1860's, and if his injury were that severe he wouldnt have been further fit for duty. Also remembering that not all injuries were combat related, especially dealing with horses.
Soldiers that were quite handy with the horse it was not uncommon to be detailed sometimes to drive wagons (teamster) or even detailed for driving an ambulance. Ambulances were generally attached to a Brigade or Division. If there was insufficent drivers, they would recruit from units within for the temp duty. If he was a cavalry trooper and found himself without a horse that late in the war, he could have been detailed elsewhere. Quartermaster Dept frequently would recruit soldiers for "extra duty" for labor and teamsters to fullfill a need. These generally recieved extra rations and/or extra pay for doing so.