Mr Reynolds was arrested during a taxi journey after a party
The Independent Police Complaints Commission (IPCC) has been told it must take full control of a probe into why a man was found in a coma in a cell.
The inquiry into what happened to Garry Reynolds in Brighton had been split between the IPCC and Sussex Police.
High Court judge Mr Justice Collins ruled the IPCC should take full control of the inquiry but could ask the force to help if it wished.
Mr Reynolds remains seriously ill in hospital with severe brain injuries.
Before Friday's ruling, the IPCC had been investigating what happened after Mr Reynolds came into contact with Sussex Police, while the force had been looking into what had happened up to that time.
However, the family wanted Sussex Police removed from the inquiry and were granted a judicial review of the case in the High Court.
Mr Reynolds, 39, from Southwick, had spent the evening of 1 March at a private party at a public house in Kemp Town.
The High Court heard that just before 0200 GMT on 2 March he was picked up by a taxi driver in Eastern Road.
The driver said Mr Reynolds had been "talking jibberish" and he took him to Brighton city centre as he did not seem to know where he wanted to get out.
He then stopped to ask for assistance from police, saying that Mr Reynolds was being threatening and his right eye was white "as if he had no pupil".
Mr Justice Collins told the court that police officers said they could smell alcohol and his speech was slurred.
They attempted to arrest him, but he struggled and "kicked out".
The judge said the police sergeant ordered Mr Reynolds to be "taken to the ground" and it was reported that a "thud or crack" was heard as if his head "might have hit the pavement".
The court was told, that according to officers, there was no apparent injury and the noise must have been caused by something being dropped.
Mr Reynolds was handcuffed and put in a cell in Brighton Custody Centre.
The judge noted that police observations over the next few hours recorded no injuries or signs of a physical or mental condition.
However, at 1100 GMT on 2 March police called an ambulance as Mr Reynolds "could not be woken up and was in a coma".
Mr Justice Collins said it was clear now that Mr Reynolds had sustained severe brain injuries which could be life-threatening.
He told the court there was concern as to whether he had been assaulted or had had an accident which had left him with the head injury.
"There may have been excessive force used at the time of the arrest which caused Mr Reynolds to hit his head," Mr Justice Collins said.
"Alternatively it may be that proper care was not taken [in the cell]."
Speaking outside the High Court, Mr Reynolds' brother Graeme said he welcomed the ruling, but would return to court if Sussex Police did have anything to do with the inquiry.
Both the IPCC and Sussex Police said they looked forward to receiving the written judgement.