Standing in for Jose Mourinho, who was sent from the bench in the second half of Real's 2-0 defeat at the Bernabeu, Karanka told a news conference UEFA's decision smacked of double standards.
He noted that European football's governing body had punished some of Real's players and Mourinho earlier this season for what it ruled was the deliberate engineering of cards, something he said was not in UEFA's statutes.
"Now they have decided not to punish Barca for something that is in their rules," he said after Real accused some of the Catalan club's players of play-acting and diving.
"Everyone saw the pictures of what happened at the Bernabeu," Karanka added.
"In football there are certain values of respect and fair play. Tomorrow on the pitch there will players who do not respect them.
"After UEFA's decision tomorrow's match is relegated to second place."
UEFA also dismissed Barca's complaint against Real over comments by coach Jose Mourinho belittling the achievements of his counterpart Pep Guardiola and suggesting there was a conspiracy among referees to favour the Catalan club.
UEFA has brought charges against both teams over the incidents in the first leg and they face a disciplinary hearing on Friday.
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