No. 5: Timofey Mozgov
The 2014-15 Cleveland Cavaliers were stuck in the mud in LeBron James's first season back, so they made a handful of roster changes partway through the year. One of those moves was to bring on board center Timofey Mozgov, a Russian behemoth who could defend the paint and free LeBron and company to play more aggressive defense out on the perimeter.
Mozgov started immediately and helped to stabilize the team, cleaning the glass and knocking bodies around. He was a problem the 2015 Warriors had to solve to win the title, eventually drawing him out of the paint and playing him off the floor. Even so, he was a very good player and a core piece of the cast around the three stars.
In 2015-16, however, the Cavaliers realized they were at their best with the more mobile Tristan Thompson at center, and LeBron James and another combo forward filling out the frontcourt (Richard Jefferson was the choice that season). They therefore did not prioritize re-signing Mozgov that summer, especially in the face of so many teams having cap space because of a spike in league revenue.
The Los Angeles Lakers swooped in and signed Mozgov to a lucrative four-year, $64 million contract...and proceeded to fall off of a cliff. His level of play spiraled and he became on of the very worst contract in the league. The Cavaliers, meanwhile, made it back to two more NBA Finals without him. It was exactly the right move to let Mozgov walk that summer.
FINAL SLIDE: The wingman