The USA international sustained a serious knee injury against Manchester United with about two months remaining last season and it was expected he would be back in action by September.
But Coyle says the 25-year old product of Houston may miss be sidelined for longer than that.
"He's optimistic and he is improving, no doubt about it, but we have to make sure we pick the right moment to bring him back and not push him at all," Coyle told the Bolton News.
"Stu has targeted the middle of September. My medical lads are a wee bit more wary because they think if he doesn't come back then I'll go off my head, and they're right because I want my players back as soon as possible."