This is a really tough one because Bill Buckner played pretty well for the Red Sox in the mid-1980's. However, he famously made an error in extra innings of Game Six of the 1986 World Series that allowed the Mets to score the winning run and force a Game Seven which they also won. The moment went down as one of the most memorable moments in World Series history and cemented Buckner as the face of Boston's failure.
What followed was pretty wild as the Boston media tore into Buckner and he received many personal death threats and threats against his family. His entire baseball career boiled down to that one botched grounder. Fortunately, this story did end up having a somewhat happy ending as Buckner was invited to throw out the first pitch when Boston put up their 2007 World Series banner and he received a lengthy ovation from the home crowd.
When the Boston Red Sox signed Hanley Ramirez to a four year, $88 million deal after the 2014 season, it was supposed to be a homecoming of sorts to where Hanley started his career. It was also hailed as a signing that could put Boston over the top as Hanley had played well with the Dodgers the previous two years when he was on the field.
Unfortunately, Hanley never really lived up to the billing of his contract as he slashed .260/.327/.450 line in 429 games after he was signed. He was a shadow of his former self defensively, his speed was gone, and he ended up getting released part way through the 2018 season. $22 million a season was a lot of money in 2014 and fans were not thrilled that it was going to a guy who struggled to be a league average hitter.