Wearing a cast covering most of his lower left arm, Jackson revealed after practice Monday he'll be out four to six weeks. Jackson hopes he'll be out no longer than four weeks, which would put him in a position to be ready for Buffalo's opener Sept. 12 against Miami.
"That's what I'm shooting for anyway," Jackson said, speaking to reporters for the first time since he was hurt in a 42-17 preseason-opening loss at Washington on Friday.
Opening the game as the starter, Jackson was hurt two plays in during a 7-yard run. That's when he landed on his hand while attempting to brace himself as he was being pulled down by the facemask by safety Kareem Moore, who was penalized on the play. Jackson said he stayed in for a few more plays before leaving the game to have his hand examined.
Tests taken over weekend confirmed that Jackson broke the fourth metacarpal bone.
"It's a huge disappointment," said Jackson, who finished last season as the Bills' starter and led the team with a career-best 1,062 yards rushing. "Standing on the sideline, you can't do much to help your team."
Jackson wasn't the only running back missing practice. Marshawn Lynch, who split the starting duties last season, walked with a noticeable limp and is out indefinitely after hurting his ankle Friday.
The Bills returned to training camp Monday, and prepare to host Indianapolis in a game at Toronto on Thursday.
Coach Chan Gailey isn't scheduled to meet with reporters until after the team's night practice.
The injuries to Jackson and Lynch leave the Bills with an inexperienced backfield. And it puts the team in a position to lean heavily on rookie first-round draft pick C.J. Spiller, who was initially expected to be eased into the three-back rotation.
Spiller's development was delayed after he missed the first seven days of training camp before signing his contract on Aug. 6.
"The mindset is when a man goes down you never want to see it, but you have to prepare yourself as if you're next in line," said Spiller, selected ninth overall out of Clemson. "Now I'm the next guy in line to try to step up into those big shoes and try to fill them for this preseason. I'm going to try to do my best."
The ACC's player of the year last season, Spiller is a speedster who proved to be a versatile threat as a running back, receiver and on special teams at Clemson. He finished his four-year college career with 51 touchdowns, including 32 rushing, and joined Reggie Bush in becoming only the second player to have 3,000 yards rushing, 1,500 yards in kickoff returns, 1,000 yards receiving and 500 yards in punt returns.
Against Washington, Spiller had four carries for 16 yards as part of Buffalo's overall dreadful performance. It was a game in which the Bills offense went 1 for 10 in third-down conversions, their defense gave up three straight touchdown drives in the first half, and their special teams allowed Brandon Banks to score on a 77-yard punt return.
"I feel a whole lot more comfortable going into this game than I did last week," Spiller said. "I've just got to step up."
The Bills were intending to use Spiller in numerous situations, including lining him up in a two-back system alongside either Jackson or Lynch.
Spiller is now expected to share the rushing duties with Chad Simpson, who previously appeared sparingly in 25 games with Indianapolis over the past two years, and undrafted rookie free agent Joique Bell. Buffalo added another running back on Monday by signing Andre Anderson, an undrafted rookie free agent out of Tulane.