The Premiership side had been in the Island for a pre-season friendly against Jersey Reds.
The incident happened on Sunday morning at the Hotel de Normandie and the member of staff was taken to hospital. He is expected to make a full recovery.
Zach Fieldsend, who was staying at the hotel with his fiancée and called the ambulance after seeing the man after he fell, said he seemed to have fallen from ‘quite a height’.
Mr Fieldsend said: ‘At about 4 to 4.30 am, my fiancée heard a huge thud outside, which woke us up.
‘We were on the first floor and looked out and saw a bloke laid on his front with his face down in the ground.
‘We ran downstairs – my fiancée is an orthopaedic nurse, so she said, ‘‘We need to get to him quickly’’.
‘The hotel was on lockdown at that time in the morning, so we couldn’t get out of the door at first. We managed to find the caretaker to let us out and we ran around to him.’
Mr Fieldsend said it appeared that the man had fallen from the second-floor room directly above his.
‘The windows open all the way out and his was wide open,’ he said. ‘I thought the worst because he was laying in an unnatural position. It was only as my fiancée went to inspect him that he started murmuring and he came back round.’
A States spokesperson confirmed the man was ‘stable’ and recovering in hospital.
In a statement, a spokesperson for the Premiership side said: ‘Harlequins can confirm that a member of staff was injured in an accident on Sunday morning in Jersey.
‘He remains in hospital there, where he is expected to make a full recovery.
‘His family have asked for privacy at this time.’
Harlequins had defeated Jersey by 33 points to 14 on Saturday evening in the Reds’ second pre-season friendly.