Dunlop was absent from Tuesday evening’s rider safety briefing for the Dundrod event, which is compulsory for all participating competitors.
Organisers have confirmed they have had no contact with the Ballymoney man in the build-up to this week’s event.
The 29-year-old – who has earned six Ulster Grand Prix wins in his career – has not raced since his brother died at the Skerries 100 event in Co Dublin last month.
Dunlop Ulster Grand Prix clerk of the course Noel Johnston said: “No-one has heard from Michael and I have not contacted him out of respect.
“The door is always open for him to compete at Dundrod whenever he decides the time is right.”