When asked if there was a chance Jackson could be back before the end of the season, Gailey said: "I don't know about that. We'll have to wait and see."
Jackson aggravated the calf in last Sunday's 35-8 loss to the Miami Dolphins, sitting out much of the second half as the injury-plagued Bills continued their downward spiral. Gailey said initial X-rays on Jackson came back negative, and Jackson told reporters after the game he would be fine. But the injury appears to be more serious than initially believed, and the team's official website suggests examinations this week revealed something that the initial tests missed.
It's a significant loss for a team that's relied heavily on Jackson's 934 yards and six touchdowns on the ground. C.J. Spiller is expected to be Jackson's primary replacement.
"Those are blue whale shoes to fill," said Spiller. "He's one of the best doing it in the league right now."
The team said Jackson's status will be re-evaluated next week.
The Associated Press contributed to this report.