That makes me think of my favorite baseball player, Ken Griffey Jr. He was the best center fielder in the league and was on pace to break Hank Aaron's career homerun record. 

Midway through his career, he started dealing with nagging injuries and never got back on track.

Undisciplined

Stacker_Sports

NBA What If's

Don't know too much of NBA. Although I have known a lot of players from Cricket who are my 'what ifs'. Now I can recall one from Newzeland—

Shane Bond.

He was the best bowler in early 2000s who could swing the ball at 140+ kph. He had nasty bouncer and also deadly Yorker in his arsenal. At his peak he troubled the best of batsmen including the greatest like Sachin, Kallis, Dravid, Steve Waugh and many more. 

However, his carreer couldn't run smooth and in 2004 he injured his knee that went through surgery but only partially successful, though he returned on field but in 2006 he had a back injury that took him almost 2 years to return to Cricket and when he returned he couldn't be 100% normal once again. So, he retired from all forms of Cricket in 2010. 