Newcastle Diamonds rider Mark Lemon will be out for around four weeks after a new collarbone break was discovered during a medical examination.
The 38-year-old had a plate fitted to support a three-way break in April, but pressure on the bone has resulted in a further break.
"This is yet another blow for Mark," said team boss George English.
Lemon completed a permanent move to Brough Park from Elite League side Poole Pirates in November 2010.
Jason Lyons will ride as cover in Sunday's visit of Plymouth, while Lee Complin will travel as replacement for the meeting at Sheffield on Thursday.