In the third inning yesterday, Mike Napoli tagged up from second base on a fly ball to right field and slid hard into third. Nobody gave it a second thought. In that moment, though, it became clear that Napoli’s first spring training with the Red Sox couldn’t have gone much better. Two months ago, there were concerns about the health of his hips. After being diagnosed with an ominous-sounding condition called avascular necrosis, Napoli endured a 51-day renegotiation in which his three-year, $39 million agreement with the Red Sox was reduced to a one-year, $5 million contract laced with incentives designed to protect the team against further injury. And now? Napoli has played in 14 Grapefruit League games, including yesterday’s 12-9 loss to the Orioles, with 38 at-bats, already one more than he had last spring with the Texas Rangers. Less than a week before Opening Day, he appears to be finding a groove, too, notching five hits over the past two days. “What went on in the offseason, I never felt anything in my hips, but everyone put that in my mind that there was something wrong,” Napoli said. “To be able to run the bases and slide and do all the things, play free, I feel like I should. Nothing’s bothering me.” Napoli’s health is essential to the Red Sox, especially because David Ortiz will open the season on the disabled list. With Big Papi absent from the middle of the order, Napoli is left as the Sox’ most feared power threat, the likeliest candidate to bat cleanup against Yankees ace CC Sabathia next Monday in New York. When he’s healthy, Napoli is a force at the plate. He’s one of six American Leaguers with at least 20 home runs in each of the last five seasons, and his career average of 15.3 at-bats per homer ranks seventh-best among active players with at least 2,200 at-bats, including free agents Jim Thome and Russell Branyan.