If humans had cellulase would they be able to digest grass?
No. Even for an herbivore, digesting grass, or plants to be more general, is hard, because they contain cellulose. Herbivores have different parts of a stomach, whereas humans only have one compartment, so in herbivores, the plants (or grass) enter the first part of their stomachs called the rumen, which contains a salty solution that breaks down cellulose, then, the herbivore will regurgitate the processed food, or cud, from the rumen, into their mouths, and break down the cellulose even MORE. The cud then enters back into the rumen and then digests as normal. So even if a human had cellulase, they would need a rumen, to break down the cellulose further.