The Spaniard picked up a booking in the first half of Madrid's 4-0 victory over Bayern Munich, which saw the club reach the final of the competition for the first time since 2002.
However, Alonso believes that it is unfair for him to miss the final in Lisbon after being booked just three times in the whole of this season's Champions League campaign.
"The rule is not fair at all, because you are playing for 13 games, three yellows cards is more than easy to be shown. It needs to be changed," Alonso told reporters.
Real will face either Atletico Madrid or Chelsea in the final.