The Germany defender went off injured during Arsenal’s 3-1 win over Stoke City on Saturday.
And Wenger now thinks he will miss the Christmas period.
“We have lost Mustafi from the game against Stoke,” Wenger said.
“Ramsey is still not ready. Mustafi out for about 21 days.”
Arsenal briefly went top of the league on Saturday before Chelsea overtook them on Sunday with a 1-0 win over West Brom.
Mustafi joined Arsenal from Valencia in a £35million deal this summer and has formed a strong partnership with Laurent Koscielny.
In his absence Gabriel is expected to get a chance while Per Mertesacker is still recovering from a knee injury.