Gay was injured Sunday in the Grizzlies' 94-91 victory at Denver when he caught an inadvertent elbow to the head from Nene. He was listed as day to day with a mild concussion, but Grizzlies coach Lionel Hollins said before Tuesday's game that Gay would not be available.
Sam Young, who started last year after Gay sustained a season-ending left shoulder injury, was expected to start at small forward.
Also, forward Zach Randolph said he hopes to return from a knee injury "shortly." Asked if it could be for one of this weekend's games against Toronto or Washington, he said it was possible.
Randolph has been out of action since tearing a right knee ligament on Jan. 1 at Chicago. He also is listed as day to day and was on the court shooting jumpers before the game, a brace on the knee.
The 6-foot-9 Randolph, who led the Grizzlies last season with 20.1 points and 12.2 rebounds per game, said the main concern at this point regarding his return was his confidence in playing again.