"No, he's not going to make it," Gailey said when asked about Jackson's chances to return before season's end. The injury takes a minimum of two to three weeks to recover from and the Bills have three games remaining on their schedule.
Jackson told ESPN that an MRI exam revealed he has a Grade 2 sprain of his MCL.
"I mean technically it's a tear off the bone, because you tear the ligament away from the bone. So that's why some people say it's a tear, but the ligament itself is intact. It's just pulled away from the bone a little bit," he said.
"It's something that I've done before, so I knew what they were talking about when they said it," he said.
Jackson was hurt with 5:26 remaining in the Bills' 15-12 loss to the Rams, when two St. Louis defenders sandwiched the player's leg while making a tackle.
Jackson was unable to put much weight on his leg, while being helped off the field. He was driven off in a cart while fans chanted "Freddie!"
The injury is the latest setback for Jackson this year, and is to the same knee he sprained in a season-opening 48-28 loss at the New York Jets. Jackson missed the next two games. He also missed a 19-14 win over Miami on Nov. 15 after sustaining a concussion a week earlier.
Jackson, who's led the team in rushing in each of the previous three years, has 437 yards rushing this season -- his fewest in five seasons. He has scored three touchdowns rushing, and also has 217 yards receiving and a touchdown.
Jackson, who signed a two-year, $9 million contract extension in May, still led the Bills with 934 yards rushing last year despite missing the final six games with a broken bone in his right leg.
His injury means the Bills will lean more on co-starter C.J. Spiller, who has 944 yards rushing and five touchdowns. The 2010 first-round draft pick is averaging 6.55 yards per carry, which is the NFL's second-best total through 13 games since 1960 for a player with a minimum of 130 attempts. Jim Brown averaged 6.61 yards per carry for the Cleveland Browns in 1963.
Information from ESPN NFL Insider Adam Schefter, ESPN reporter Josina Anderson and The Associated Press was used in this report.