Michael Dunlop has revealed that he suffered a broken pelvis and other fractures in Thursday's crash at the Southern 100 on the Isle of Man.
Dunlop, 30, came off in the Senior Race and missed the feature class after being taken to Nobles Hospital on the island before later being discharged.
The Ballymoney rider now looks a major fitness doubt for next month's Ulster Grand Prix.
There will be racing on 8 August, with the main Dundrod race day on 10 August.
Dunlop, who gave details of his injuries in a tweet on Friday morning, was forced to miss the feature Solo Championship race which concluded the Southern 100 meeting.
19-time TT winner Dunlop was running just behind leader Dean Harrison when he crashed at Castletown Bridge on the second lap of the Senior race.
The 30-year-old also crashed out of the Donegal International Rally and revealed he had shoulder and wrist injuries after that mishap.
Dunlop was second in the Open race on Tuesday behind Harrison, finishing 10 seconds down on the Kawasaki rider.
Dean Harrison made it six wins in a week at the meeting by winning the Solo Championship race, having also triumphed in the Senior race and three Supersport 600cc outings.
Bradford rider Harrison became the third most successful rider in the Southern 100's history, with his six victories during this week's racing taking his tally to 23 overall.
The Silicone Engineering Kawasaki rider also became the third rider to enjoy success in the Solo Championship three years in a row, joining Joey Dunlop (1976-78) and Guy Martin (2013-15) in that elite club.
Dom Herbertson doubled up in the Supertwins ahead of Michael Sweeney.