What species is it? Spearers that live in burrows such as Lysiosquillina and Alachosquilla often lie on their backs. This is less common in smashers. That the animal is eating is a good sign, but I honestly can't predict the outcome.
Let me try one other tact. Stomatopods, unlike crayfish, lobsters and you and I, do not have an organ that responds to gravity and tells them which way is up. Rather, they rely on a dorsal light response. Simply put, they balance the light coming from above striking their two eyes and this generally keeps them upright. You can check this out by putting an animal in an aquarium or plastic bag in a dark room and shining a light from below. They will flip over on their back - putting the light on the top half of the eyes and balancing the input. When an animal loses an eye, it only gets input from one light sentivie organ. They will turn or roll towards the side with the eye trying to balance the input. It doesn't work so they turn in circles or roll over on their sides. (This was some of the first research I did on stomatopods in the mid-60's) After a few weeks the brain will re-wire and they will start to orient normally. While stomatopods can regenerate most appendages, they cannot regenerate an eye. Instinkt, your animal will always be a cyclops. It may, however, regenerate a short antennule in the place of the missing eye. I have found several animals in the field with this happening.