The Boston Red Sox got a key piece of their bullpen back Saturday when they activated reliever Garrett Whitlock following a short stay on the 10-day injured list. Whitlock was put on the injured list Aug. 13 with right elbow inflammation that he did not think was serious at the time. “A high level back end arm, that’s one of the premier setup relievers in the game,” interim manager Chad Tracy said before the Red Sox faced the Yankees in a day-night doubleheader.