These are only small percentages of the season. But nonetheless, the Red Sox offense has fallen off a cliff. And not just because of Cliff Lee.
Red Sox offense from June 3-20:
.310/.389/.523 with 22 home runs and an average of 8.0 runs per game. They were 14-2
Red Sox offense from June 21-28:
.254/.333/.347 with 3 home runs and an average of 2.3 runs per game. They were 1-5
The pitching hasn't been all that different. The staff had a 3.28 ERA during the hot streak and a 4.32 during the last six games.
There is no particular reason for the drop-off in offense. Mostly it's just the rhythms of the season combined with some poor situational hitting, missing David Ortiz for a few games and Jacoby Ellsbury having a .222 OPB for a week.
In time, probably soon, the Red Sox will kick their engine into gear.