The 21-year-old Sweden international limped off during a 1-0 defeat to Werder Bremen in Germany on Sunday and Cottagers boss Martin Jol is waiting on the results of scans.
The Dutchman said: "Hopefully it is not too bad but he may be out for a couple of weeks.
"That is not what you want from pre-season. Kaca has made a lot of big steps and progress with us."
On the prospect of bolstering his strikeforce ahead of their Premier League opener at Sunderland on August 17, Jol added: "We will try to get someone up front.
"We need a few more goals, since Clint [Dempsey] left we do not have enough goals in us, so we are working every day to get someone. We have some high-quality players in consideration and we would like another midfielder as well."