No, you can omit them and the soup will still be great, but make a few changes. 1) You won't have any homemade mushroom stock, so use 4 cups (960 mL) vegetable broth instead of 3 cups (720 mL). 2) To compensate for the loss of flavor from the mushroom stock, I recommend adding 1/2 teaspoon porcini mushroom powder when you add the vegetable broth (affiliate link). It adds great savory depth of flavor to all kinds of soups, stews, and lentil dishes. I haven't tried this product, but a few of my readers have used Trader Joe's multipurpose umami seasoning as a substitute for porcini mushroom powder in other recipes. 3) If you want to add extra mushrooms (since you're not adding the rehydrated mushrooms), use 20 ounces (570g) of fresh mushrooms instead of 16 ounces. But this is totally optional.