There is no fear of Jose Iglesias being undone by a lack of confidence. The Red Sox third baseman is fighting to remain in the major leagues, but he carries himself with the swagger of an established veteran. Last night offered a demonstration of Iglesias’ self-assurance. It wasn’t just that he went 2-for-3 — with singles against a dominant CC Sabathia — in the Red Sox’ 4-1 loss to the Yankees. It was the way Iglesias acted after the game. As a swarm of reporters approached his locker, the 23-year-old asked them to wait until he finished an ice cream bar. He then disappeared for about 10 minutes to enjoy the treat before returning to a nearly empty clubhouse. Those are the things you can do when you have a .435 batting average. “He’s been confident both times he’s been with us this year,” Red Sox manager John Farrell said. “He believes in himself, feels like he can excel at this level. He’s doing that, and he’s doing a very good job both defensively and at the plate for us.” The field is never in question for Iglesias. His glovework has made him a tantalizing prospect since the Sox signed him in 2009. The natural shortstop has seamlessly shifted to third base since being called up May 24 to replace Will Middlebrooks, who is on the 15-day disabled list with a lower back strain. The revelation this season has been Iglesias’ production at the plate. The necessary disclaimers are that it’s a small sample size and his extraordinarily high batting average on balls in play will inevitably drop.