No, you can also freeze the cabbage instead of boiling it. Remove the core from the cabbage, then place it in the freezer the night before. In the morning, move the cabbage to the refrigerator, so it's defrosted for that evening.  The freezing process breaks down the cabbage enough that it will be easy to work with. When you cook the rolls, either slow cooking or in the oven, the cabbage will cook the rest of the way along with the filling.