The 42-year-old was taken to hospital after fracturing the femur in his left leg and his collarbone and suffering serious injury to the fingers on his left hand. He had been testing his new Ilmor bike.
The official MotoGP Web site www.motogp.com said McWilliams would undergo surgery on Monday.
Spanish rider Dani Pedrosa clocked the fastest time of the day on his new Honda as riders tested out their new 800cc bikes for the next season.
Five times MotoGP and 500cc world champion Valentino Rossi, who was fastest on the first two days of testing on his Yamaha, posted the second quickest time.
