Cassidy said Sunday that the swelling hasn't gone down enough to make a definitive diagnosis, but added "it's probably not going to be good news." Carlo was hurt Saturday when he landed awkwardly on his leg in the third period of a 5-1 win over Florida.
"You feel awful," Torey Krug said of his defensive partner. "It's a tough break, but I know he's got a lot of support and a lot of good people around him."
The short-handed Bruins did get defenseman Zdeno Chara back for Sunday's game at Philadelphia after a nine-game absence because of an upper-body injury. Chara was paired with Nick Holden, who moved to the right side.
Krug played with Adam McQuaid.
Center Riley Nash didn't travel a day after needing more than 40 stitches in his ear, head and neck after getting hit with a puck during the game against the Panthers. He could rejoin the team for games in Tampa Bay and Florida this week.
Cassidy said defenseman Charlie McAvoy skated Sunday morning and could return Tuesday. He has been sidelined 14 games with a sprained left knee ligament.
The Bruins also are hoping forward Rick Nash (upper body) could return this week.
Despite a long list of injuries all season, the Bruins entered Sunday atop the Eastern Conference with 109 points.
"When you think you're finally maybe turning the corner, but I guess this is just how we're going to be," Cassidy said. "It's become a bit of our fight song, so to speak."