"I am happy to announce that I am leaving for Japan," said Andrea Caldarelli, who was holidaying in Italy when he got the surprise call-up.
He said he will be racing "car 36", which is the Petronas TOM's car usually raced by Lotterer.
So far, however, Caterham is quiet. "We currently have nothing to say on this question," a spokesperson is quoted by Speed Week.
Behind the scenes, however, team sources are unofficially admitting that Kamui Kobayashi will not be in the green car in Belgium this weekend.
Speed Week claims the only thing holding up an announcement is the lodgement of Lotterer's contract with the FIA lawyers in Geneva, and - as a Le Mans winner and past Formula Nippon champion - the formality of his F1 super license.
It is expected the announcement will be made by Caterham's new management early on Wednesday, perhaps along with confirmation that a sponsor, the energy drink Hype, is also signing up.
Germany's Auto Motor und Sport said Lotterer's Le Mans team, Audi, has agreed to release him from his contract for the Spa race.
Audi spokesperson Eva-Maria Veith told Spox: "There was a conversation.
"It is up to the team to announce whether or not it is happening, but we have no problem."
At the moment, Japanese Kobayashi is scheduled to return to the Caterham cockpit in Monza.
Lotterer is fighting for the Super Formula title, and in September his Audi duties in the world endurance championship will resume.
Lotterer's manager Werner Heinz is quoted by Spox: "The negotiations are not yet complete, but it looks good."