Boyd faces six weeks on the sidelines as he needs a hernia operation.
Boyd is set to go under the knife on Tuesday after consulting a specialist and could miss the Gers' next nine games.
Boss Walter Smith said: "It's disappointing news, especially given how well Kris has been playing. He's rarely injured so it's a real blow.
"We already have injuries and when you consider the number of goals Kris has been scoring, you can see this is a very serious blow to take.
"But we just have to get on with the games and hope he can get himself back in action as quickly as possible."