Miner turned switch for batteries on Horizontal, sitting on 4ft elevated surface. Bank of surface sloped 1:1. Miner stepped back and lost footing. Miner slipped down bank, put left arm out to brace. Miner heard pop, experienced shoulder pain. Initial injury occurred on 12/14/20. Simple strain diagnosis and no time loss. On 5/27/21 Miner approved for surgery, therefore reportable.