It is the GM's responsibility to address the issues and to try to correct them, but I don't think he can be held responsible for the way the players have underperformed. If Ben had signed Clayton Kershaw last offseason, and then for some inexplicable reason he had an ERA of 6 this season, would that be Ben's fault? I know that Kershaw would be a much more extreme case, and I know that our pitchers had more question marks, but the underlying point remains the same.
I don't think Ben was gambling on everything working out in the Sox favor, but nor was he expecting almost everything to work against us. The staff was not supposed to be great, but mediocre. They have been worse than that. I don't think the staff is the biggest reason why this team is in last place. I still say that if the offense had been performing to expected levels all season, the Sox would be leading the division.
Some analysts might have thought the team was poorly constructed, but most that I read thought the Sox would be competitive. More analysts had the Sox winning the division than any other team. The computer models all favored the Sox. On paper, this team should be leading the division.