The striker has struggled with injury niggles since the start of pre-season and only trained for the first time this summer last Friday.
It was hoped that Okenabirhie would play some part in Sunday’s friendly at Harrogate Town but the setback means he is some time away from a return to action.
“Fej has had a setback and he will be a while longer,” boss Richie Wellens said.
“He’ll be more than a doubt for the start of the season but we don’t know how long yet.”
John Bostock is set to resume full training on Monday after suffered knock-on effects from a Covid-19 diagnosis.
The midfielder tested positive for the virus during the period in which multiple senior players were isolating due to close contact with a case.
His recovery took longer than the standard ten day isolation period and the Rovers medical team are being cautious with his return.
“Bostock was back in training on Wednesday,” Wellens said.“Because he showed symptoms for longer than his initial isolation period, he has to go through a few tests to make sure there’s no underlying things.
“He’s come through them and he’ll probably be back in full training on Monday.”
Ben Close missed Wednesday night’s friendly with Sheffield United after contracting Covid-19 at the weekend.
He is due to return to training next week in time to prepare for the opening day match against AFC Wimbledon.
Wellens said: “It was the Saturday morning after Newcastle, he didn’t feel too well. He had two tests and they both came back positive.
“Everyone was okay. In terms of close contact, there was only Tiago Cukur because he travels in with Ben but he’s had Covid recently so he doesn’t need to self-isolate.”