The Australian suffered the problem during the Carling Cup clash against Nottingham Forest in midweek and had to be withdrawn before half-time.
Gary Megson admits it is a major blow to lose him at this stage of the season.
"He's had a scan, it's a bad pull, and it could be four to six weeks," said the Leicester boss.
"It's bad news for us and bad news for Jimbo. His career has been blighted by injuries. But he's a tough lad and he will be back," he told the Leicester Mercury .