Matthew Carley was due to take charge of Saints' clash with Bristol, but he has been forced to withdraw due to illness.
He has been replaced by Tom Foley, who was the man in the middle for the black, green and gold's most recent match.
Foley sent off Api Ratuniyarawa during the 20-16 defeat to London Irish on January 24.
The referee was far from popular on the evening, with the home fans roundly booing the official.
They had earlier been frustrated that Irish scrum-half Ben Meehan was only yellow carded for an off-the-ball tip tackle on Cobus Reinach.
And that frustration was exacerbated when Ratuniyarawa was dismissed after his shoulder made contact with the head of Irish prop Harry Elrington at a ruck.
Saints went on to concede two tries in the final seven minutes as Irish earned a shock win in Northampton.
Foley is set to be assisted by Karl Dickson and Wayne Barnes this afternoon, with Graham Hughes the television match official (TMO).