Worcester Warriors scrum-half Shaun Perry is facing six months on the sidelines after a scan on his injured knee revealed cruciate ligament damage.
Perry, 33, was injured during the first half of Friday's Premiership win over Northampton Saints, although he went on to play the remainder of the match.
"He needs an operation and will be out for six months," head coach Richard Hill told BBC Hereford & Worcester.
"It's a huge blow for the club and a huge blow for him as well."
Hill added: "He had been playing really well and had won two man of the match awards from the five games he had played.
"To lose a player of that quality is a real blow but Ollie Frost will now come in to start at the weekend."
Perry's natural replacement would be Jonny Arr but he is also out of action because of a medial knee ligament problem and is likely to be missing for another four weeks.
Hill said the search for an experienced number nine to come in as cover was already underway.
"If Shaun does what Jake Abbott did last year and comes back in April and May to make the end of the season that would be terrific," Hill said.
"But in the meantime we are going to have to find a replacement for him.
"There is a World Cup on at the moment and one or two players might be available but we are going to have to search every corner we can do to try to find the most suitable replacement."
Former England international Perry joined the Warriors on a one-year contract last summer after a two-year spell in France with CA Brives.
With Frost due to start at London Irish on Sunday, Hill confirmed that Danny Gray is likely to be used as cover at scrum-half.