The Belgium international was disappointed with his side's showing against the Rojiblancos but believes they still have a great chance of progressing to the knockout stages
Porto took a 16th-minute lead through Jackson Martinez, but the Portuguese champions were unable to build on that advantage as strikes from Diego Godin and Arda Turan saw the visitors emerge victorious.
However, Defour believes that Zenit St Petersburg's inability to beat Austria Vienna in the group's other game on matchday two handed his side an early reprieve.
"I think it was a tight game, with Atletico sitting very deep, not allowing us to find space and create opportunities. We were prepared for that kind of strategy,” the 25-year-old told reporters after the game.
"Our first half hour was good and we applied lots of pressure. After the equaliser, our level dropped and ended up conceding another goal.
"However, Zenit's result turns out to be good for us, so we must take full advantage in the next two games against the Russian team. All in all, the group is wide open."
Defour's midfield team-mate Josue bemoaned Porto's defending from set-pieces, as they allowed Atletico to score twice from set-pieces.
"We suffered two goals in dead-ball situations,” he said. “The second one cannot happen. Now we have to work in order to make amends in our next game."