Tiger Woods' decline happened after he'd already firmly established himself as at least the second best golfer in history without question. So... no. It's not the worst in professional sports. How about Derrick Rose, who was poised to become the next great thing, and even won an MVP award, only to deal with injuries and eventually getting traded, with people figuring he'll never regain his top form? How about Penny Hardaway, who was poised to be the next big thing post-Jordan before he got injured and his career completely fell apart? Picking someone who is unequivocally the second best ever as the "worst decline" just seems absurd given all the people who were heralded as the next big thing only to become at-best borderline hall-of-famers. Hell, look at Len Bias; he was going to be the next big thing and died before he even went pro. That's a much worse decline than "maybe he's better than Jack Nicklaus... no, he's not."