Goals from Gokdeniz Karadeniz and Pablo Orbaiz gave the visitors an unexpected victory in the first leg of their clash with the reigning champions of the competition.
Falcao, who won the trophy with Atletico last season and Porto the campaign before, believes he and his team-mates lacked luck in the last 32 bout.
"It was a very strange game,” he told reporters. "They created their goal and ended well.
"But they hardly had any space to create opportunities and attacked very little. We lacked their fortune."
The Colombian striker refused to criticise Diego Simeone's decision to send up goalkeeper Sergio Asenjo for a corner late in the game, which was the cause of Rubin Kazan’s second goal, and remained optimistic ahead of their away leg.
"Those situations are decided on the field and now we have to think it over," he added. "I believe we can overcome the deficit, it is not impossible."