How to add missing garlic flavor to cooked soup
I wouldn't use raw garlic, as it has a much different flavor from cooked garlic.

If you have some roasted garlic, you can put it in a blender with some of the liquid from the soup, then mix it into the rest of the soup.  If you then let it sit overnight, it should help to spread the flavor.

Another alternative (although it won't necessarily replicate the original) would be to make a gremolata (garlic, parsley and lemon zest) and drop a small spoonful into each bowl when serving.