Federici, who had played every minute of Reading's Championship campaign this season, had to be replaced late in the Watford clash after sustaining the injury when he twisted his knee diving for a ball.

The Australian goalkeeper was on crutches and in a knee brace after the game and the severity of the injury was unclear.

But the Royals confirmed on Monday he would be out for a matter of weeks after receiving the results of scans performed on Sunday.

The injury, though, will see Federici miss the Royals' exciting fifth-round FA Cup tie against Chelsea's conquerors Everton on March 1.

It also means Federici, who opted to miss the 2011 Asian Cup, faces a race against time to be available for selection for Australia's March 29 friendly against Germany in Moenchengladbach.

Reading physio Luke Anthony told the Reading Post: "The MRI scan confirmed what we first thought and Feds has suffered from a medial ligament injury.

"It's impossible to put a time on it at the moment, we'll just have to wait and see with him. But right now it looks like he will be out for weeks instead of months.

"He is in good spirits. He's had a pretty clean injury record in the past so it's all a bit new to him. Now we just have to get him on a recovery plan and work from there."