Hillsborough match commander David Duckenfield was "basically a spectator" in the police control box on the day of the disaster, a court has heard.
Former police sergeant Michael Goddard told Preston Crown Court in practice, it was ground controller Bernard Murray who would have run the operation.
He said Mr Duckenfield could not be commander "because he was so new", having been in the role three weeks.
Mr Duckenfield, 74, denies the gross negligence manslaughter of 95 fans.
Mr Goddard, who had been operating radios in the control box at the FA Cup semi-final on April 15 1989, was being questioned by Benjamin Myers QC, defending Mr Duckenfield.
Asked if the match would be a steep learning curve for a match commander doing the job for the first time, Mr Goddard said: "An impossible learning curve."
He agreed a chief superintendent had to be in the police control box at a match of that size, but as a "matter of rank rather than experience".
The court heard there were five officers, including Mr Goddard, Mr Murray and Mr Duckenfield, on duty in the control box on the day, as well as Robert McRobbie, a chief inspector who was there to observe.
Asked if Mr Duckenfield would have been reliant on other officers, Mr Goddard said: "Absolutely. He was a bit like Mr McRobbie, basically a spectator."
Mr Myers asked: "Does it follow that once he was in position of being match commander, David Duckenfield was going to rely very heavily indeed on Bernard Murray?"
Mr Goddard replied: "Totally."
He agreed that all minds in the police control box were focused on resolving the crush outside the turnstiles, and nobody foresaw any danger that would follow on from Mr Duckenfield's order to open exit gates to the ground.
He said: "I as an individual and I as part of a team let Mr Duckenfield down there, I wish I'd have done better."
Being re-examined by Richard Matthews QC, prosecuting, Mr Goddard said he was not aware of Mr Duckenfield's experience of policing Hillsborough as a chief inspector in 1979 or of meetings he had attended about policing football in South Yorkshire.
Mr Goddard accepted he had no experience or knowledge of exactly what was required of an officer in the role of match commander.
Mr Duckenfield, of Ferndown in Dorset, denies causing the deaths of 95 Liverpool fans who died in the fatal crush on the Leppings Lane terrace.
Under the law at the time, there can be no prosecution for the 96th victim, Tony Bland, as he died more than a year and a day after the disaster.
Former Sheffield Wednesday club secretary Graham Mackrell, 69, denies breaching a condition of the ground's safety certificate and failing to discharge a duty under the Health and Safety Act.
The trial continues.