Keller, who has played abroad longer than in other American in a career that started in England in 1991 and has included stints in Germany and Spain, is one of four Americans on the Fulham squad.
With Brian McBride also injured (knee), Fulham is down to two healthy Americans (Carlos Bocanegra and Clint Dempsey).
Keller's biceps injury may require surgery.
"I feel absolutely destroyed, devastated," said Keller in the Daily Mirror. "It's hugely disappointing because things were going so well and then this comes along and now I have to start all over again. I just wanted to make my name here and I was doing that until I suffered this cruel injury."
Keller was signed as back-up to No.1 Antti Niemi.