As the Red Sox have endured their longest losing streak in three seasons in the past week, all Alex Bregman has been able to do is watch from the sidelines. Bregman, who has missed just over a month with a quad strain he suffered on May 23, has resumed baseball activities over the past couple days, swinging a bat and taking grounders on the Fenway Park field. “It’s very tough,” Bregman said Saturday. Bregman, who outplayed his massive free agent contract with a .299 average, 11 homers, 17 doubles and a .938 OPS in 51 games to start the season before getting hurt, is now unquestionably Boston’s best offensive player with Rafael Devers having been traded to the Giants.