5. Mark Blount
The second-round draft pick in 1997 had to fight his way into the NBA. He did not play his first game until 2000 with the Celtics. Blount was a reserve big man for two years in Boston before signing with the Nuggets in free agency. The Celtics traded for him in February 2003 and signed him to a massive contract in the summer of 2004 after a breakout fourth season where he averaged 10.3 points, 7.2 rebounds, 1.0 steal, and 1.3 blocks in 29.3 minutes per game as their starter.
Blount never lived up to his six-year $41 million contract. His defense and rebounding immediately dropped off. He would be ripped for his lack of effort, and the seven-footer virtually dropped out of the rotation in the 2005 playoffs as the Celtics were bounced by the Pacers in the first round.
Boston traded Blount to the Timberwolves in a seven-player deal for Wally Szczerbiak in 2006. He continued to score, but never showed the same defensive prowess as his contract year. He produced a zero VORP in 2005 and was negative in each of his final four NBA seasons. Blount made nearly $7 million per season and produced like a minimum player.
The Boston Celtics certainly regretted giving him that contract and never should have gambled that much money on him after one season of production.