Police chief Troy Finner said officers had interviewed the guard, who said he was hit in the head, fell unconscious and woke up at a medical tent.
The initial allegation led officials to look into a possible drug-spiking incident at the Travis Scott festival.
Eight people were killed and hundreds others injured after a crowd surge.
Speaking at a news conference, Chief Finner said organisers were told by police to end the event on Friday night when one person in the crowd was being given cardiopulmonary resuscitation.
But he declined to provide an exact timeline of how events unfolded.
Chief Finner estimated the investigation, which is being led by the Houston police department, could take weeks or months to complete, and said he did not believe an independent probe was needed.
There have been concerns about possible conflicts of interest as the police department deployed about 500 officers to the concert, and because of the relationships Scott, who is from Houston, has with local authorities.
Chief Finner dismissed those worries, saying he had only met the singer twice and that there was no “close relationship” between the two.
The Houston Police Department and the FBI are investigating the incident, which Finner estimated could take weeks or months to complete. They are working to review video footage and interview victims as well as witnesses. Finner said Wednesday that he does not believe an independent investigation is necessary at this point.