Hartnell had one assist in three games this season. He posted 37 goals and 30 assists last season.
In the wake of Hartnell's injury, the Flyers, who posted a 2-1 victory over the Rangers on Thursday night, signed veteran winger Mike Knuble.
Knuble's one-year deal is worth $750,000, a source told ESPN.com's Pierre LeBrun.
The 39-year-old Knuble had six goals and 12 assists in 72 games for the Washington Capitals last season.
Information from ESPN.com's Pierre LeBrun and Scott Burnside and ESPNNewYork.com's Katie Strang was used in this report.