I don't think they will necessarily need a total rebuild, but they probably will (say what?). They're going to fire the GM, who will fire Crennel and likely bring in a coach who doesn't run the same system. That means a lot of players and personnel who don't match what they do right now. They could sign someone who wants to keep doing the 3-4 thing, but that could go either way. They won't need to totally blow up the team, but it will be a few years.
I want to take this time to thank John Elway for bringing in John Fox, getting us to the playoffs that same year, and then signing Peyton Manning the year after and bringing in Del Rio. I definitely think with the right guy, the Chiefs could be back in a couple years. We lucked into bad division in the first year of our rebuild. The Raiders and Broncos won't be crappy next season so the Chiefs will need that extra year, but two years, maybe three - if they land a decent GM and coach - and they can get things righted.