Lee tweaked his left ankle late in Boston's 104-94 loss to the Dallas Mavericks on Friday, but it's unclear when Garnett suffered his own injury.
Saturday will mark just the fourth missed game of the season for Garnett, who had just returned from a left adductor strain and a bout of the flu that kept him out of consecutive games last week. Celtics coach Doc Rivers told reporters before Saturday's game that Garnett would likely undergo an MRI for the injury.
Lee, meanwhile, will miss only his first game of the season on Saturday.
For Memphis, Marc Gasol is out indefinitely after aggravating an abdominal tear in Friday's loss to New Orleans.