Usyk, 32, amassed all four world titles at cruiserweight and his move up a division has been highly anticipated.
But Surinamese-Dutch fighter Spong’s position is in doubt after Voluntary Anti-Doping Association (Vada) testing.
“We have reserve opponents standing by,” tweeted Hearn.
“We have been contacted by Vada to inform us that there has been an adverse finding in Tyrone Spong’s test. It has been sent to the State of Illinois commission.”
Hearn said there will be an update on Tuesday.
Usyk is scheduled to face Spong at Chicago’s Wintrust Arena in his first bout since stopping Britain’s Tony Bellew in November, when he defended all four world cruiserweight titles.
Spong, 34, spent much of his combat career in mixed martial arts and also competed in kickBoxing before winning his first 14 fights as a boxer.
Vada operates in Boxing and MMA and requires fighters or promoters to pay for testing for the duration of a fight build-up. It said Spong was part of its testing programme on 20 September.
The Las Vegas-based Vada has earned a glowing reputation in Boxing and was responsible for tests which led to the likes of American heavyweight Jarrell Miller and Mexico’s Saul ‘Canelo’ Alvarez facing bans for doping in recent years.