It isn't that Scutaro is irreplaceable, the problem is well beyond that. By replacing Scutaro with Aviles, they lose a reliable source of production and a good clubhouse guy, but it is not the end of the world. But then on top of that, instead of having Aviles as a backup, the team's utility guy is Punto who has hit to a .650 OPS for his whole career, and averages 1 home run per season.
It also sends the message that the Red Sox are sellers. That despite having fans sellout out their stadium for years on end, they can't afford to pay their starting shortstop? This organization can have no justification for trading Scutaro unless they use that money for something else more important.