Gay first injured the ankle Dec. 16 in the Grizzlies' 110-97 loss to the Atlanta Hawks. He left the game but played in the four games since, averaging almost 19 points over that span.
He reaggravated the injury Saturday in a loss at Dallas.
Gay, who averages 20.6 points and 6.1 rebounds, had started all but one game this season: Nov. 30 at Utah. He was out for personal reasons.
DeMarre Carroll, a rookie out of Missouri, started his first NBA game in place of Gay. Carroll averages 3.6 points and 2.6 rebounds in 27 games.