Roger Federer is going to miss the U.S. Open and be sidelined for months because he needs a third operation on his right knee, a procedure he said will leave him with “a glimmer of hope” that he can return to competition. Federer announced the news Sunday via a video message on Instagram. “I’ve been doing a lot of checks with the doctors, as well, on my knee, getting all the information as I hurt myself further during the grass-court season and Wimbledon,” Federer said. “Unfortunately they told me for the medium- to long-term, to feel better, I will need surgery, so I decided to do it. I will be on crutches for many weeks and then also out of the game for many months.” The 40-year-old Federer, who has 20 Grand Slam singles titles to share the men’s record with Rafael Nadal and Novak Djokovic, acknowledged there was a …