The Red Sox roster underwent a significant change Monday that went beyond third baseman Will Middlebrooks being activated from the disabled list. Jose Iglesias is going nowhere. The 23-year-old Cuban, bitterly disappointed when he was demoted to Triple-A Pawtucket back in April, is on the big-league roster for the forseeable future, and in a role that goes beyond being a spare part. Manager John Farrell said that Iglesias will play at least three times a week on the left side of the infield, which means he’ll be sharing time at third with Middlebrooks and on occasion, perhaps once a week according to Farrell, will spell veteran Stephen Drew at shortstop. The Red Sox announced just before game time that infielder Pedro Ciriaco, who is not here with the team, has been designated for assignment to make space on the roster for Middlebrooks’ return. Middlebrooks was in Monday’s lineup against Tampa Bay, batting eighth and playing third. The Sox had come into this season maintaining they preferred that Iglesias play every day rather than keeping him on the bench with the big-league team. But Iglesias’s performance while Middlebrooks was on the DL with a lower back strain obviously altered the club’s thinking. Iglesias is batting .444 (24 for 54) since his recall May 24, with 5 doubles, 2 home runs and 10 runs scored. He has played superb defense at third base, a position he had never played before at any level until last month, and is a known commodity at short, where there has never been any question that he is a Gold Glove-caliber fielder. Iglesias’ bat was the issue, and as recently as five weeks ago, when he was benched in Pawtucket for a desultory attitude, questions abounded about whether he would hit. At the time of his recall, he was batting .202 for the PawSox.