League One rivals Rotherham United, meanwhile, are waiting on scans for centre-back Michael Ihiekwe and goalkeeper Viktor Johansson.
Signed from Portsmouth in the summer, Hiwula picked up the injury in last week's friendly against Sheffield United.
“He is going to be out for quite a considerable amount of time,” said manager Richie Wellens
“He got a bad tackle against Sheffield United with about two or three minutes of the half to go. He carried on but we’ve had it scanned and it’s not good.”
Okenabirhie has aggravated the foot problem he started pre-season with.
The fear is that an injured quad muscle could put Ihiekwe out for six weeks. Reserve goalkeeper Josh Vickers is expected to miss a month with a similar problem, which made Johansson limping out of Saturday's friendly against Middlesbrough even more of a concern, though his substitution was described more as a precaution.