Well, looking at it from a straight probability standpoint, assuming all outcomes are a 50/50 chance, you have a 25% chance of winning the series if you drop Game 1, and a 75% chance if you win game 1.
So, while you're not totally dead in the water if you don't win Game 1, it's definitely a big deal. Obviously the odds aren't exactly that and there are many variables in play, but it gives you an idea.