The 6ft 6in lock signed a three-year contract to move from his native South Africa in September, but an Achilles injury suffered in training shortly after his arrival has kept Steenkamp sidelined ever since.
The initial prognosis given by head coach Steve Tandy was an absence until March, but Rees has revealed that is now unrealistic.
"He will start next season," Rees told the South Wales Evening Post.
"We'll build him up for the latter part of pre-season, and hopefully he'll be available for the World Cup period."
Steenkamp's signing was described as 'significant for the region' by rugby general manager Andrew Millward and though his prolonged spell on the sidelines has not proved detrimental on the field - with Ospreys third in the GUINNESS PRO12 table - they have suffered two defeats in their last three matches.
Steenkamp is not the only absentee from the Ospreys boiler room as they prepare to take on league leaders Glasgow Warriors on Saturday.
Alun Wyn Jones is away on international duty, while Rynier Bernardo and Lloyd Peers sit alongside Steenkamp in the treatment room, so rookie Rory Thornton is in line to make just his second Ospreys start.
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