Lincolnshire's Gary Johnson and Manxman Conor Cummins will miss this week's Ulster Grand Prix at Dundrod as they continue their recovery from injuries.
Johnson suffered a broken collarbone, a broken shoulder and four chipped vertebrae in a crash on the exit of Ramsey Hairpin in the Superstock TT.
Cummins fractured his forearm while riding his Jackson Racing Supersport Honda 600cc bike at the Southern 100.
Cummins is a former winner and lap record holder at the 7.4-mile circuit.
The 28-year-old was scheduled to ride a fleet of Hondas at the event but his crash at the Southern 100 meeting in mid-July has put pay to his chances of adding to his solitary victory in the 2009 Superbike race.
Johnson, 34, was hoping to secure his first-ever Grand Prix success, having previously finished second on three occasions.
The English rider took his second Isle of Man TT Supersport win in June, before coming to grief in the Superstock outing.
Among the top riders taking part at Dundrod this week are the Dunlop brothers, Michael and William, 11-time winner Guy Martin, lap record holder Bruce Anstey and in-form Yorkshireman Dean Harrison.
In the absence of Cummins, Dan Kneen will fly the flag for the Isle of Man on board his Cookstown BE Racing machines.
British Superbike rider Peter Hickman, who made spectacular debuts at the North West 200 and Isle of Man TT, rides a BMW, while Ian Hutchinson will hope to make an impact on his Milwaukee Yamahas.
Lee Johnston and Jamie Hamilton join the Dunlop boys in leading the Northern Ireland challenge, while Keith Amor makes a comeback to the event and ex-Moto3 rider Danny Webb and British Supersport regular Glenn Irwin make their Dundrod debuts.