Substitute for Rose Water? It only calls for a tsp and it's an 8 serving lamb shank recipe. Will it matter if not included?

I'd just leave it out, since it's such a small amount. I'm sure it adds another layer of flavor to the dish, but I doubt you'll miss it if you don't have it. Depending on the other ingredients, you could add a little citrus (lemon, orange, or kumquat) to give it interest in the same manner.