The 22-year-old damaged a cruciate ligament against non-league Arnold Town on Wednesday.
County manager Shaun Derry told the club website: "We're going to lose Jimmy for the whole of the season because he's done his cruciate ligament and as it stands it's not good.
"It's a major blow for us all and it's come to that stage now where we're looking for some sort of replacement.
"Jimmy's been absolutely fantastic for us and I feel heartily sorry for him first and foremost.
"As a person, I feel so, so sorry for Jimmy but we'll support him and I feel that he belongs with us.
"We'll support him to make sure he comes back bigger and better than before."