Reece picked up the injury during the Raiders' 24-14 defeat to the Washington Redskins on Sunday, a match that saw his side blow a 14-0 lead from the first quarter.
An MRI scan on the 28-year-old's knee came back negative, and Allen believes that it will simply be a case of waiting for the injury to heal.
"It's really a matter of when's he going to be physically able to go. It's not a surgical procedure or anything like that that needs to be done. It's just, whenever he's ready to go," Allen told reporters.
The Raiders will be looking to improve upon their 1-3 start to the season when they take on the San Diego Chargers on Sunday.