Cassel sprained the knee during the opening series of a pre-season contest against Seattle on Aug. 29.
Coach Todd Haley said he remains optimistic about Cassel's availability.
"We've got a couple of more days of practice here to evaluate where he's at," Haley told reporters after Cassel wore full pads at Wednesday's practice, but appeared to be unhampered by the injury knee during individual passing drills.
Cassel, obtained with linebacker Mike Vrabel in an off-season trade with New England, signed a six-year contract in the off-season, believed to be worth close to $30 million.
In 2008 in place of the injured Tom Brady (knee), Cassel, a seventh-round pick by New England in the 2005 NFL Draft out of USC, passed for 3,693 yards, 21 touchdowns and 11 interceptions in 16 contests for the Patriots.