Matsui should be a solid performer from now until the end of the season unless he's hiding an injury. See Eric Chavez, Victor Martinez and Vernon Wells for how quickly fortunes can change in a baseball season. Just last week, people were asking if they should drop Victor Martinez. Now, he's a hot commodity again. Always trust your stars (and Matsui certainly is a star) to come through.
Matsui is the same hitter he always was. Just look at the first 3 games of the season. He'll come around in time.
You said it yourself, his value can't get any lower, and you can't drop him, so just hold onto him. If you have a viable substitute, plug him in there until Matsui starts hitting again. He'll come around sooner or later.
[size=10]"Men are apt to mistake the strength of their feeling for the strength of their argument." [/size]