Cook won a unanimous points verdict against Arthur to claim his title
Nicky Cook has been forced to pull out of his WBO super-featherweight title defence against Stephen Foster Jr in London on 6 December because of flu.
The Dagenham fighter had been set to make his first appearance since winning the belt from Alex Arthur in September.
Cook, who was due to meet Foster at the Excel Arena, said: "I'm absolutely devastated that I won't be able to defend my title for the first time."
Wales's Enzo Maccarinelli's now tops the bill against Johnathon Banks.
Cook added: "I returned to training last week but it was clear that I had missed too many training sessions to catch up and I just felt really weak because of the flu.
"I can't let my heart rule my head and it's important that I'm 100% fit and ready to defend my title rather than lose it and come out with excuses."
Maccarinelli is returning to action after losing the WBO cruiserweight belt to David Haye in March. He meets America's Banks for the now-vacant title.
The main supporting bout on the bill will be Amir Khan's lightweight comeback clash against Irishman Oisin Fagan.
Promoter Frank Warren was philosophical about Cook's withdrawal.
"Obviously this is unfortunate, but these things happen in boxing and Nicky needs to be fully fit to defend his title," he said.
"Enzo now headlines the show and is in a real hard fight against Banks.
"We have worked hard to get him a world title opportunity on his first fight back and now it is down to him to do the business in the ring.
"There's a lot of interest in Amir's big return and to see how he comes back after his first loss [to Colombia's Breidis Prescott in September].
"He has gone to America and trained with one of the best in Freddie Roach and we will see how he has progressed."