Wait, Gobble! Don't lay that egg just yet! Remember what they say about 'easy to add, hard to remove' - that's especially true with an egg.
I think the sour cream will be a sufficient binding agent.
I like to experiment making patties, too. All kinds, not just ground meat. On more than an occasion, I've made the mistake of adding an egg. The result - indeed, a loaf. A mixture which can only be saved by an oven.
If you add the egg, be sure to beat it in a separate bowl, and add to your mixture in measure at a time, conservatively until you have the right patty consistency.