Somerset captain Marcus Trescothick will miss Sunday's CB40 semi-final against Durham after scans revealed he has damaged ligaments in his ankle.
The 35-year-old was forced to retire during Monday's CB40 win over Essex.
He had been undergoing treatment for an ankle problem since twisting it during Somerset's FL t20 win over Nottinghamshire earlier this month.
"In the short term it doesn't look very promising," director of cricket Brian Rose told BBC Radio Bristol.
"Even with slight ligament damage it can take two to three weeks, if it's serious damage it's five to six weeks.
"We'll assess on Saturday... if there's potential for him to play towards the end of the season then we might have a look at it but in the short term, certainly in the next week I doubt if he'll be available."
When asked if the former England opener would be fit for the CB40 final, should they beat Essex, Rose added: "It's 50/50 - that's the target."
Trescothick has already been forced to sit out Somerset's Division One clash with Hampshire.
His side are currently fourth in the County Championsip, 21 points behind leaders Lancashire, who they have to play in their final game, with three matches left.