Hathaway didn't participate in Thursday's practice session due to an illness, Jordan Hall of NBC Sports Philadelphia reports.
Hathaway has appeared in 47 games for the Flyers this year, recording a goal, an assist, 183 hits, 52 PIM and 35 blocked shots while averaging 10:17 of ice time. He'll have nearly a week to recover before Philadelphia faces Washington on Wednesday, so it wouldn't be surprising to see him available for that matchup.