Who is the shortstop target in this theory? This all would have made a lot of sense if Jose Iglesias was still on the team. Reports on Marrero don't make him out to an exceptional defender.
I'm not sure I agree that his defense isn't exceptional. Here's his scouting report from sox prospects.com
Scouting Report and 2015 Outlook: Marrero is extremely athletic with great instincts. Defensively, he has an active lower half with agility, quick lateral movements, and soft hands. Marrero is a plus-plus defender with plus range and a plus arm. He makes playing the position look easy, getting to balls deep in the hole and possessing plenty of arm strength to make all the throws. He has great body control and is able to throw from all angles and on the run. Offensively, the right-handed hitter displays a line-drive stroke with quick hands through the zone and a gap-to-gap approach with solid knowledge of the strike zone. He has below-average raw power, which is fine for a shortstop. He is a solid baserunner with instincts and average speed that can steal a base. Given his approach at the plate, athleticism and ability to make adjustments, I believe Marrero can offer future average offensive output on par with other Major League shortstops. Marrero offers premium defense at a premium position with a chance to provide average offensive output from the shortstop position, and even though he has already reached Triple-A, he will not turn 25 until next August. He should begin 2015 back with Pawtucket and could make a push as early as 2016 for the everyday starting job at shortstop, potentially