Arsenal manager Arsene Wenger dropped Gallas from the team which lost 3-0 to Manchester City in the Premier League on Saturday and the Frenchman has now been stripped of the captaincy.
Wenger told a news conference on Monday that Gallas would return for Tuesday's Champions League Group G match against Dynamo Kiev.
Asked if he would be captain, Wenger replied: "No it will be Fabregas."
He declined to elaborate on Gallas's demotion but confirmed that Fabregas, 21, would be a permanent appointment.
"William is a player that I rate. He was working as a captain in a very difficult media environment and was under pressure from the press," Wenger said.
"I do not to have to explain to you why I make the decisions. When you don't win nobody jumps off the roof and the frustration kicks in when your team doesn't win.
"This is a good opportunity to show that we have strength and ability. You know how the media works, you go from catastrophe to fantastic.
"Even if we are the moment frustrated it's not all doom and gloom. We have a strong belief in our ability and we have a good opportunity to show we care about the club.
"We believe in our strengths and we believe we can qualify tomorrow."
Gallas's comments four days ago, in which he accused his team mates of "not being brave enough in battle", came during a troubled season for the north London club who slipped out of the top four after Saturday's defeat, their biggest league loss since March 2007.
Theo Walcott, one of their best players this season, has been ruled out for three months with a dislocated shoulder, and Wenger has been coming under increasing pressure from Arsenal supporters who believe the Frenchman is obsessed with style at the expense of substance.
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