By Leslie Patton Bloomberg November 17, 2011 Sept. 16, 2012, is D Day -- or at least K Day -- for Green Mountain Coffee Roasters Inc. (GMCR) That’s when it will lose the main patents on K-Cups, the coffee pods that helped make Green Mountain the largest player in the $1 billion-plus U.S. single-serve coffee market. Once the patents expire, competitors can make their own version of K-cups and threaten the prospects of a company already battling criticism from hedge fund manager David Einhorn that its market-share gains have peaked. Einhorn, president of Greenlight Capital Inc., declined to comment for this story and won’t say if his firm is shorting the stock. “New entrants into private label could hurt their volume,” said Bea Chiem, a San Francisco-based analyst at Standard & Poor’s , who gives the company a B+ credit rating, four levels below investment grade. When the patents run out, it could also “open up the door for other ...