Horwill was due to make his long-awaited return from a hamstring injury prior to the final pre-season match with the Chiefs just over a week ago but he rolled his ankle in training and was forced to watch from the sidelines.
He also sat out the tournament opener last Saturday as the Reds were downed 24-6 by the Brumbies in Canberra.
Reds boss Ewen McKenzie initially suggested that the 27-year-old second row would be back in time for the home tie with their biggest rivals on Saturday but he has now explained that is no longer the case.
"He's not quite ready yet so rather than mess around there we'll just continue on," said McKenzie.
"The plan at the moment is he'll probably run the water for us this week so his return is not far away.
"It's the start of a long season and we don't feel it's the right time to run guys who are semi-fit."
Horwill hasn't played for club or country for nine months now after tearing his hamstring in May of last year.
Having been appointed Wallaby captain in August 2011, Horwill hasn't worn the Green and Gold of Australia since the end of that year.
He missed the summer Tests with Scotland and Wales, the entire Rugby Championship campaign, the third Bledisloe Cup clash with New Zealand and the recent tour of Europe.