The England Under 21 international striker picked up a knock in the FA Cup Third Round game against Derby at the weekend, and was thought to have enough time to recover from the ankle injury sustained.
However Bent has not progressed as quickley as it was hoped, and is now racing a fight to be fit in time for this weekends game, and is yet to train this week.
Speaking to the Official Club Website, Town boss Joe Royle confirmed: "Darren is behind schedule in his recovery on what we expected and at the moment, is our one main worry for the Reading game,"