Gallas and Silvestre sidelined for three weeks
Silvestre limped off during last weekend's FA Cup victory over Portsmouth with a thigh injury, while foremr club captain Gallas has developed a hamstring problem.
Fortunately for the Gunners' Ivorian centre-back Kolo Toure has recovered from a groin problem and should return for Saturday's Premier League match with Bolton Wanderers at the Emirates Stadium.
"Kolo will be back available," Wenger told Arsenal TV Online. "However, unfortunately we have lost William Gallas with a hamstring problem.
"We lost Silvestre too with a thigh problem last week as well. So we have lost two players from the game on Saturday, but we have gained two back.
"I think Gallas and Silvestre will be out for three weeks."
Johan Djourou or Alex Song will most likely partner Toure in the centre of Arsenal's injruy-hit defence against Bolton.
Gallas was stripped of the captaincy in November for criticizing his teammates' fighting spirit in an interview and striker Emmanuel Adebayor has now backed the Frenchman.
"There is some truth in what he said," Adebayor said. "Someone had to say these things, to make each person face up to their responsibilities. If he had said nothing, where would we be today?
"Gallas has always been like that - he has experience, he has won titles, played in a World Cup final. He doesn't throw these thoughts around lightly. He said something because it was necessary to do it."