After returning to practice Wednesday for the first time in nearly three weeks since spraining his right knee, Jackson said the next test is seeing how his leg feels once he wakes up Thursday.
"I felt OK about practice today. That was good," Jackson said. "I think what's going to determine more than anything is how I'm feeling like the next day. If I can come out here and continue to work, that'll help get me better. But if I wake up tomorrow and it's swollen or too sore to do anything, then that's going to set me back."
Despite his concerns, Jackson stuck to the prediction he made on Monday, when he said he's 70 to 75 percent certain he'll be ready to play on Sunday.
The Bills (2-1) certainly could use him in preparing for an early-season showdown against their AFC East rival Patriots (2-1). With Jackson out since a season-opening 48-24 loss at the New York Jets on Sept. 9, Buffalo's running attack took another blow after C.J. Spiller hurt his left shoulder in a 24-14 win at Cleveland last weekend.
Spiller missed practice on Wednesday, and is considered to have an outside chance to play against New England. Cornerback Leodis McKelvin did not practice with what the team listed as a groin injury.
Jackson said he felt only a little pain after practicing on a limited basis. He did experience some difficulty while cutting on the turf, and said he also needs to become more comfortable wearing a brace.
The Bills have relied on their running attack to win the past two games. Buffalo ranks third in the NFL with 534 yards rushing.
Though initially projected to miss about a month, Jackson has been eager to return earlier in part because of the injury to Spiller and a chance to face the Patriots.
"It's the Patriots, we need all guys on deck to go out there and get a win for us," Jackson said. "It's a big game for us. But at the same time, I've got to be smart and make sure I'm out there able to make plays for my team."
Tashard Choice, who had 91 yards rushing against Cleveland, would take over as the starter if Jackson and Spiller can't play.