I think that they thought they had plenty of depth. Price, Porcello, Rodriguez, Buchholz & Kelly were going to be the starting 5 barring injury. It was made public that Kelly was promised that he was going to start the season in the rotation based on the end of 2015, plus he made it clear that he didn't want to pitch in relief.
They knew they had Wright, Owens, Elias & Johnson, all guys who had big league experience as starters. I doubt that there were many free agent "depth type starters" who looked at Boston as a "great opportunity."
Rodriguez gets hurt, Kelly gets hurt, Owens regresses, Elias regresses and Johnson has a mental breakdown. End of depth & a journeyman like O'Sullivan ends up making starts in May.
I think it was easy to question the "quality" of the rotation heading into the season, but it's tough to question the "quantity," in my opinion.