CORTINA D’AMPEZZO, Italy — The United States team for next month’s Olympics lost one of its top medal prospects Tuesday when downhill racer Breezy Johnson said injury will force her to miss the Beijing Games. Johnson placed second behind Olympic champion Sofia Goggia in each of the three World Cup downhills she started this season, then crashed in training runs before two races in January. “I have to announce that I have unfortunately injured my knee and must withdraw from the Olympics,” Johnson said on her Instagram account. The 26-year-old racer crashed in training in Cortina d’Ampezzo, Italy, last week, one week after skipping a downhill in Austria to let a cut and bruised knee heal. Johnson wrote that in Cortina she “immediately felt a massive crack in my knee” and had dislodged cartilage. “I was given the option to try to compete on it.