FYI, that little round metal piece is more decorative than functional. Some say it retains the chain of it falls off the sprocket....I say is is bendable in your hand, it can't function as a guard, as it isn;t strong enough. I've seen chains come off, they drop to the axle bolt anyway. If your chain is loose enough to come off the sprocket, the problems you are having will not be remedied by a little piece of circular tin.
Mine is off, and I am not worried. There is also a guard at the front, and if your chain breaks and the sprocket drives the chain into that area, the guard is just one more piece of metal that is crushed and broken. They serve more as oil spray catchers, like fenders.
I don;t miss mine, and wouldn't bother putting that back on except for looks.