Timonen is being treated in his native Finland and there is no timetable for his return.
The 39-year-old signed a one-year contract extension with the Flyers in June. He has played in Philadelphia since being acquired in a draft day deal with the Nashville Predators in 2007, appearing in 97 percent of the Flyers' games since.
Last season, Timmonen tallied 35 points in 77 games.
Competing internationally for Finland, Timonen has four Olympic medals and three World Championship silver medals.
- Dan Gartland