By pitching well in games that don’t count Michael Pineda will open the season in the Yankees’ rotation as the fifth starter. How long the right-hander remains at the back end of Joe Girardi’s starting five is a mystery because the Yankees are ignoring a time-honored tradition of not falling in love with positive results in March. When camp started Girardi admitted he didn’t know what he had in Pineda who hadn’t pitched in a big league game since 2011 due to tricky shoulder surgery. Now, after four games (three starts) in which the 25-year-old acquired from Seattle before the 2012 season, went 2-1 with a 1.20 ERA and whiffed 16 and issued one walk in 15 innings, Girardi has selected him over David Phelps and Adam Warren. “He threw extremely well and it was what we wanted to see from him,’’ Girardi said of Pineda, who underwent surgery on May 1, 2012 to repair an anterior labral tear in his shoulder. “He improved in each outing and at times was dominating and we really liked what we saw.’’ Pineda will remain behind in Tampa and throw Sunday. After that he will start Saturday against the Blue Jays in Toronto following Masahiro Tanaka’s Yankees debut Friday night versus the Blue Jays. CC Sabathia, Hiroki Kuroda and Ivan Nova will face the Astros in the first three games of the season. “I grew up a lot,’’ the 6-foot-7, 265-pound Pineda said of his two years away from the major leagues. “I am a better person right now. I learned a lot.’’ The Yankees are banking on Pineda being more mature. He showed up for his first Yankees camp 30 pounds overweight, hurt his shoulder because he was out of shape and then in the summer of 2012 was busted for DUI.