Bill Buckner
Sentiment and professional sports go together like water and sodium. Unfortunately, nobody told John McNamara because he cost the Red Sox a World Series. McNamara consistently brought the veteran Buckner into games throughout their playoff run as a late replacement (via SBNation).
Then he decided to keep Buckner in the final match of the World Series. He thought the Red Sox had an insurmountable lead. But Buckner made a career-defining error as he allowed a ball to bounce between his legs. Many Boston fans resented him but it was McNamara’s fault for putting him in the position.