Jackson, 31, a sixth-year pro, suffered a ligament sprain in his right knee during the Bills' game Sunday against St. Louis. He injured the same knee and missed two games earlier this season. He missed another game recently because of a concussion.
Gailey, speaking Monday on his weekly radio show on WGR-AM, Buffalo, N.Y., said Jackson would be out a "minimum of two to three weeks."
There are three weeks left in the regular season.
Jackson is the second-leading running back for the Bills this season, gaining 436 yards on 115 carries. He had three rushing touchdowns. He also caught 34 passes -- tied for fourth most for Buffalo -- for 217 yards and an additional touchdown.
C.J. Spiller is the team's leading rusher, gaining 944 yards on 144 carries.