Okenabirhie has suffered a setback in his first week of training in an injury-disrupted pre-season and now seems certain to miss the opening League One game, at home to Wimbledon on August 7.
“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.”
The 25-year-old forward, who joined in January 2020, was Doncaster's top-scorer last season with 11 League One goals.
Another who will miss Sunday's friendly at Harrogate Town is Bostock, whose recovery from Covid-19 has been longer and more complicated than expected.
He is back in training, but Rovers are being understandably cautious about his return.
“Bostock was back in training on Wednesday,” confirmed Wellens.“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.”
Doncaster are hoping summer signing Ben Close will be back in training next week after missing the midweek friendly defeat to Sheffield United with the virus.
Doncaster fans will also miss Sunday's match with no away supporters allowed at the all-ticket friendly at Wetherby Road.