Lottery pick rookie contract is guaranteed.

Yi was picked first round, so his first contract was guaranteed with length and money, and that is also strictly defined by the league and union bargain. Once the rookie contract expires, if you are really good, you will normally be offered a so-called max contract. If you are not living up to the expectation, you will normally become a journey man - getting smaller and shorter contracts from various teams. 

Yi wasn't a total bust, but obviously he's not good enough. After so many years, his problem is still defense - the willingness to defend, the toughness to defend, and the technique and strength to defend. He doesn't have the first one, not to mention other elements. What a waste of physical gift - he's long and nimble.