There are still things Greg Monroe needs to work on. His game is fitting better by the day with the Celtics. But the hat. It’s a problem. Perched on the upper shelf of his locker in the visitors dressing room at the United Center late Monday night was a blue baseball cap with the intertwined “NY” facing outward. That’s right, a Yankees hat. Monroe laughed. He explained that he just wanted to wear something blue. When informed the Red Sox cap is also navy blue, he said the red in the logo wouldn’t work with his outfit, which, of course, featured those fashionable ripped jeans. Monroe contended he has no baseball favorite — he was born in New Orleans, home of the Triple-A Baby Cakes — but eventually let on that the “Bronx Bombers are back.” He laughed again when told he should keep the Yankees lid tucked away when walking around Boston. Yet, despite what the locals would term a character flaw, the bet is that Celtics followers will forgive his diamond discretion if he keeps playing the way he has in the past three games. After struggling early as he tried to find a rhythm with his third team this season, Monroe has made 17-of-25 shots (68 percent) and averaged 14 points against Charlotte, Houston and Chicago. His best outing came against the Rockets when the Celts pounded the ball to him inside and he emerged with 18 points, including a 6-for-6 effort from the free throw line, in 19 minutes. “It’s just getting into a rhythm, getting comfortable,” the 6-foot-11 Georgetown product said. “I’m just trying to be aggressive and make the right play. That’s it.” Monroe began the season in Milwaukee, got traded in November to Phoenix and reached a buyout agreement with the Suns on Feb. 1. The Celts signed him as a free agent into the disabled player exception granted for Gordon Hayward’s injury.