You could just play Mankind divided if you really wanted to, but Mankind divided takes place almost directly after the ending(s) of Human Revolution so the set up of whats going on and why its going on is something you might want to see first hand. Also the main character Adam Jenson is the lead of both games so it might be good to play Human Revolution so you can get acquainted with him, though pretty much every other character in human revolution doesn't make an appearance in mankind divided. Also many people think Human Revolution is the better game.