According to FIFA rules, a challenge of the like Nani engaged in with Madrid's Alvaro Arbeloa should only merit a straight red card if it is deemed to have been executed with "excessive force."
Nani's did not, nor was it "violent conduct." He was airborne for the purpose of controlling the ball and only aware of Arbeloa's challenge at the very last moment—if at all.
His challenge was, at the very most, "reckless." You could argue it was made with "complete disregard to the danger to, or consequences for, his opponent." FIFA rules dictate that reckless challenges are punishable by yellow cards.
Had he been booked, Manchester United would more than likely have beaten Madrid and advanced to the Champions League quarterfinals. Cakir's red card turned the tie in Madrid's favor and ensured the opposite happened.