"Cesc will miss the remainder of the domestic campaign as he will be out of action for at least six weeks," a statement on the club's website said.
The Spain midfielder injured his right fibula in a collision with Barcelona defender Carles Puyol in Wednesday's 2-2 Champions League quarter-final first-leg draw in London.
He picked himself up to tuck away the resulting penalty for Arsenal's equaliser and hobbled through the final eight minutes before leaving the stadium on crutches muttering that he feared the worst.
"I hope he recovers as soon as possible so we can start enjoying his play again and he can go to the World Cup because he will be a very important player for us," Barca captain Puyol told a news conference on Thursday.
Fabregas, who has been in sparkling form for third-placed Arsenal, has scored 19 goals in all competitions this season, and was patched up by Arsenal's medical department this week after receiving a kick against Birmingham City on Saturday.
"It was a gamble worth taking," Fabregas said after an epic encounter with his old club, although manager Arsene Wenger may well be questioning his own judgement now, especially with William Gallas and Andrei Arshavin suffering calf injuries.
Wenger, who will hope his walking wounded can keep the team's title-tilt going, also have defender Gael Clichy (back) and midfielder Denilson (groin) struggling for Saturday.
"At the moment, we are like an army who have come back from a big battle," Wenger said.
"We have some bruises but we need to recover and then go to the next battle. Who will be available for the next one? I have to wait 24 hours more."
