The Norwegian defender was forced off during Monday's 4-1 win over Crystal Palace with a nerve problem.
Manager Martin Jol now admits that the centre-back could be sidelined for the Cottagers' next game.
"He is injured," Jol told reporters. "He's got a nerve that caused him problems.
"He played two games for Norway so I thought he was 100% but at half time we had to take him off.
"It's a sciatic nerve from his Achilles. I'm not a doctor but a nerve can cause you problems."
Monday's victory over Palace saw Fulham climb up to 14th place in the Premier League table.