It's a combination of three things... the bad weather which caused some games to get postponed, his high on base percentage (and large number of walks) and the management decision by the Red Sox to give him plenty of rest.
I don't expect him to sit all that often, maybe once every other week. It gives a player who is perceived as being injury prone time to rest his body and it also gives some at bats to Wily Mo Pena, a player that the sox want to develop.
I wouldn't be concerned. Batting fifth behind Ortiz and Manny, I think he'll be a valuable fantasy player all year.