Yoan Moncada’s long-awaited Boston Red Sox debut has gotten off to a slow start. The third baseman came into the second half of the Major League Baseball season as the league’s top prospect, so the expectations understandably were high when he made his first appearance on Sept. 2. Moncada was hot, too, going 4-for-10 with a double, an RBI and three runs in his first three games. Then the 21-year-old showed his biggest weakness: strikeouts. Moncada made the jump from Double-A to the bigs despite striking out in 30.9 percent of his at-bats in Portland. And after posting a golden sombrero in Tuesday’s 5-1 win over the San Diego Padres, Moncad... NESN.com