The club initially anticipated a four to six-week lay-off for the 19-year-old after he injured himself landing awkwardly in a tackle during the 2-0 Carling Cup loss.
Further examinations have now shown the injury to be worse than thought and the former Crystal Palace playmaker is likely to be out until spring.
Schwarzer looked certain to quit Craven Cottage after expressing a desire to join Arsenal, who had more than one bid rejected for him in the summer.
But the 38-year-old Australia goalkeeper, whose previous deal expired next summer, has now committed himself to Fulham until the end of the 2011-12 season, with his new deal containing the option of a one-year extension.
Wenger maintains his side have the quality needed to come good in the end as they look to secure a first trophy since the 2005 FA Cup by staying on course for Wembley with victory against Wigan at Emirates Stadium on Tuesday.
"For over two months we have played every three days, and part of that is sometimes you have ups and downs. It is how you respond to that," Wenger told Arsenal TV Online.
Thousands of Blues fans charged towards the Villa supporters and dozens of missiles and some flares were thrown before order was restored and City can expect to face Football Association action.
Zigic's deflected shot five minutes from time settled the tie after Gabriel Agbonlahor had cancelled out Sebastian Larsson's early penalty.
Williamson has been instrumental in Newcastle making a solid return to the Barclays Premier League with manager Chris Hughton rating the 27-year-old as one of the finest central defenders in the top flight this season.
Former Portsmouth defender Williamson told the club's official website: "Playing in the Premier League with a club like Newcastle has been a dream for me, and I was delighted to agree a new contract to continue doing that until 2016."
