Diaz has averaged under five innings per start this year. Assuming six per start is very generous. You cannot speculate how many innings he will throw out of the pen by checking the most amount of innings one of our relievers has thrown in a month. Diaz's arm is conditioned for many more pitches per appearance than any of our relivers. He threw four innings in relief earlier this year, none of our 'long relievers' have come close to that.
Grilli has shown more composure than Diaz in his starts, and has actually improved in each consecutive outing. Diaz allows a home run almost every three innings pitched. Grilli has earned himself the fifth starter's spot for the rest of the season.
Also, Garcia is ready to start again. Letting him throw will not further injure his arm (at least in the way it was previously sore). He didn't have to miss as many starts as he did, but the Sox were very precautious.