The Portugal international was dejected after hearing the bad news but insisted he will work hard on regaining full fitness
The 28-year-old tore the anterior cruciate ligament in his right knee on Monday in the club's mid-season training camp in Florence and is expected to be sidelined for the next eight months.
Danny, who will miss the rest of the season, as well as the Euro 2012 finals, flew to Portugal on Tuesday to be examined by physiotherapist Antonio Gaspar, who confirmed the initial diagnosis.
"There is no doubt that I am sad and disappointed, not only to miss Euro 2012, but also the Champions League and the Russian Premier League," he told reporters.
"But this is the first day of my recovery and I have to think positive."
The Portugal international, who joined Zenit in 2008, spent 10 months on the sidelines in 2009 after suffering a similar injury to his right knee.