Mortensen, 26, was a first round draft pick of the St. Louis Cardinals in 2007, but has struggled ever since reaching Triple-A, and was designated by assignment in January before being dealt to the Rockies. While he had some decent results in a short stint in the majors last year, his peripherals suggest that was a fluke. Given his track record, it's hard to see Mortensen as anything more than minor league depth well past the likes of Cook and Silva.
Marco Scutaro, on the other hand, has managed to solidify Boston's shortstop position for the last two years after five years of chaos. In two years with the team, Scutaro averaged a line of .284/.343/.401 producing 5.3 fWAR despite playing injured for much of 2010. At the moment, Nick Punto and Mike Aviles seem likely to take over his role in 2012.
By trading Scutaro, the Sox have freed up more than $5 million in salary, presumably to pursue a starting pitcher like Roy Oswalt. While any addition to the rotation at this point will be more than welcome, it's hard to imagine that the Sox couldn't find any sort of return for a reasonably cheap, league average player at a position of some scarcity.