Wright had been due to challenge Theophane at the Peterlee Leisure Centre on Saturday, live on Sky Sports HD1.
However Wright has been replaced on the card by Southern Area lightweight champion Murphy after ruled out due to medical reasons.
Promoter Frank Maloney said "Murphy is in great shape he's been live in the gym and deserves a crack at the title for his professional approach to boxing and I believe this will make a great fight on Saturday.
"I feel sorry for Nigel Wright but I don't think we could have got a more prepared replacement in Ben Murphy."
The 31-year-old Murphy has a mixed record of 8-4-1 but he does hold a win over current British super-featherweight champion Gary Buckland.
And Murphy, who claimed the Southern Area belt on November 25 by stopping the previously unbeaten Tony Owen in three rounds, insists he is ready to step in and upset the odds.
Murphy said, "I'm in the shape of my life and raring to go. Theophane won't know what's hit him come Saturday night.
"Since winning the Southern Area title I've been down the gym training hard for a chance like this and I couldn't believe it when I got the call and had no hesitation in taking the fight.
"My trainer Johnny Eames told me opportunities like this come around once in a lifetime, and this is why I must always be prepared."