Having spent most of the offseason transforming his body, John Lackey insists he’s now ready to alter his mindset, too. “I feel like it’s my first year in the big leagues again,” he said yesterday. Sixteen months removed from surgery, Lackey has a rebuilt right elbow and another chance to remake his image with the Red Sox. After missing all of last season, he is set to reclaim his spot in the starting rotation and to prove he’s a better pitcher than he’s demonstrated since signing a five-year, $82.5 million contract three years ago. But why should anyone believe in Lackey, whose 6.41 ERA in 2011 ranked as the worst ever by a Red Sox starter? For one thing, he finally is healthy after at least one year of pitching through a shredded ligament in his elbow. “It had been a little while,” Lackey said of the last time he felt healthy. “I don’t know exactly, but probably a couple years. It’s just nice for it not to hurt, to be honest with you. I definitely feel like (surgery) took a few years off.” And since the end of last season, Lackey has taken off about 20 pounds, too. As manager John Farrell said this week, Lackey has “smoothed out the lumps” in the hopes of pitching more like he did during the second half of the 2010 season, when he posted a 3.97 ERA and 99 strikeouts in 102 innings after the All-Star break. “It was just a matter of putting in a lot of work,” Lackey said. “I rehabbed my arm last year, and I wanted to give myself the best chance I can to perform well.”