Conor Cummins has has broken his forearm following a four-rider spill in the Supersport race at the Southern 100 on Wednesday evening.
Honda Racing's Cummins and Tom Parkes were airlifted to Noble's Hospital following the incident on the opening lap at Ballakeighan.
Parkes has since been discharged, while Ramsey man Cummins is now described as 'comfortable'.
Cummins has been back to his best this season and clinched a superb runner-up finish in the Senior race at the Isle of Man TT.
He was also the leading 600cc rider in Tuesday's 1000cc/600cc Cortlett's Trophies race at the Southern 100, which was won overall by Dean Harrison.
James Cowton and Ivan Lintin were also involved, but escaped largely unscathed.