The 29-year-old, who admitted that he would be "extremely disappointed" to miss out on the game, picked up a virus at the start of the month and lost five kilograms in a week as a result of the illness.
"I've never been so ill in my life," BBC Sport quotes Haskell as saying. "They thought I had meningitis and put me in an ambulance.
"They knew it wasn't bacterial meningitis but they thought it might be the viral version because I had all the symptoms of it. The big thing for me was not eating. I normally eat five or six times a day but I was struggling to get one or two meals in.
"I'm sure when I'm back in full training mode I will pick the weight up again pretty quickly. I don't foresee it as being an issue."
The flanker returned to training on Tuesday.