The Court of Arbitration for Sport says that a decision by the Spanish athletics federation and Olympic committee to exclude Mullera "did not constitute 'technical reasons'."
The Spanish team dropped Mullera last week while it considered disciplinary proceedings against him for alleged doping.
The federation was reportedly told by an anonymous source about alleged email exchanges between Mullera and a sports trainer indicating he may have been doping or was planning to dope.
The CAS says Mullera's Olympic registration had never been withdrawn by the Spanish team.
Steeplechase heats begin Friday.