The veteran frontman, who spent the end of last season on loan at Glanford Park from Sheffield United, was replaced at half-time during Saturday's 3-3 draw with Bournemouth after picking up the problem.
"Geoff's really sore and undergoing treatment at the moment," said manager Peter Jackson.
"We anticipate two weeks. It might be sooner, it might be later, we'll have to assess it as the week goes on."