Mildenhall Fen Tigers are looking for a replacement for rider Aaron Baseby less than a week after re-signing him.
The 22-year-old broke his ankle in a crash during last weekend's 45-45 National League draw with Stoke.
"It was a heavy tumble. He's in plaster and it looks like he's going to be out for at least six weeks," co-promoter Michael Lee told BBC Radio Suffolk.
Baseby was brought back to the club from Isle of Wight Islanders after Josh Bates, 16, fractured his arm.
The latest injury was ill-timed for the club with the second leg of their Knockout Cup semi-final, also against Stoke, this Saturday evening.
"We're busy searching for a new reserve at the moment. There are riders out there but we're looking for someone who's capable of picking up a few points," said Lee.
"It is difficult because the people out there are very inexperienced and to bring them in at this stage of the season, they'd find it very tough.
"We're also looking to when Josh will be back in action - I know he's keen to get back on the bike.
"We don't want him back too early and cause any more damage but I think he's talking about a couple of weeks, so it may be we don't have to cover for too long."