more than any of the other 4 major sports in the US (key: just because cricket/(football and yes this is our soccer)/etc aren't popular in the US doesn't mean they aren't world-wide sports) Tiger singlehandedly carries a league by himself....lets look at the other sports, shall we?
MLB: No single player is THAT identifiable (A-Rod, Pujols, Big Papi, Griffey Jr. are all identifiable, but not nearly the same level as Tiger...plus, with it being a team sport it does not single an individual like golf does)
NFL: Lots of well-known talents, like Brady, Mannings, Urlacher, etc etc but even if you remove any one of them, the league still functions as an entity and does not lose a fanbase/numbers stay about the same...
NHL: The remaining followers of the NHL wouldn't care if Hossa, Crosby, Ovechkin, etc weren't playing, they still would enjoy the game...while Crosby/Ovechkin (sry for butchering his name) weren't there, casual fans may leave, but the core would remain...no one nearly as important here in their sport more than Tiger
NBA: Lebron, Chris Paul, Amare, T-Mac, Big 3 (KG, R. Allen, The Truth), Kobe, Shaq....you get the idea...even with any of these players (including PLENTY I HAVE LEFT OFF, not a slight to any of them these just popped into my head) the league as a whole wouldn't drop drastically...yea Lebron, Kobe, and perhaps C. Paul would have more influence than some others, but you all get the idea....
and just to throw it out there, NASCAR (quickly rising in popularity overall in US, i believe much moreso now than hockey)...remove Dale Jr, arguably the most identifiable man in the sport, and again, the fan base doesn't change/sponsors find other players.....NOW....
focus: without Tiger, PGA loses an enormous amount of casual fans, as well as a ton of sponsorships who are looking to reach the younger crowd....with Tiger gone for the rest of the year, I think it is safe to say we will see a significant drop-off of ratings and perhaps endorsements within the sport (hard to gauge those numbers, as they are probably hard to obtain anyways)....in conclusion of my massive ramblings, I do believe Tiger is among the greatest, IF NOT THE GREATEST, at what he does in sports history...just my 2 cents