I'm really not sure exactly what the Red Sox need to to, outside of resolve first base.
Their starting pitching is good. Their lineup, position-by-position is good. Their bullpen is solid, today's game not-withstanding. Who do you really replace?
Personally, I would try to get a solid defensive catcher, move Martinez to 1B and Youk to third. You can keep Lowell as a part-time DH with Ortiz and backup to Youk. This way, it wouldn't cost you anything and you'd shut down the running game of your two biggest opponents, the Angels and Yankees.