Michael Raffl, the fourth-line left winger who was the Flyers’ top offensive performer in Sunday’s 4-1 upset of Boston, is sidelined indefinitely.

Coach Alain Vigneault wouldn’t speculate on how much time Raffl would miss, but said rookie Joel Farabee would be in the lineup Thursday when the fourth-seeded (for now) Flyers play third-seeded Washington in their second round-robin game in Toronto.

Raffl had a goal and an assist in the win over the Bruins. The good-natured Austrian appeared to injure his left leg and was helped off the ice with 6 minutes, 17 seconds left after being knocked awkwardly into the boards by defenseman Jeremy Lauzon.

“I can’t get into specifics. I can’t get into lengths because of NHL protocol,” Vigneault said in a Zoom call with reporters Monday afternoon. “I can say he won’t be available for a little bit.”

Raffl, 31, is also used on the Flyers’ effective penalty-killing unit.

“I think we have some depth this year on the penalty kill. We have a few rotations going,” center Sean Couturier said. “We’ve used a lot of guys on the penalty kill, and it’ll just be another guy stepping up and getting the job done.”

Raffl (two points), Nate Thomson (goal), and Tyler Pitlick (four hits, drew a penalty) formed the Flyers’ best line Sunday.

Farabee has been used up and down the lineup, but he will likely fill Raffl’s fourth-line spot against the Caps, though there is a chance he moves to the third line and James van Riemsdyk is demoted to the fourth unit. Vigneault was not happy with van Riemsdyk’s play in the exhibition win Tuesday against Pittsburgh or the round-robin victory Sunday against Boston.

Vigneault said he was “expecting more” from van Riemsdyk.

Couturier said the Flyers will miss Raffl’s presence on the ice and in the locker room, but he was hopeful he wouldn’t miss a lot of time.

“He’s a great guy to be around. He’s loved in the dressing room, and were going to miss him if it’s for an extended period of time,” Couturier said. “But there are some other guys waiting to get an opportunity. We have some depth, which is a good thing, and we can use that.”

Pitlick agreed, calling Farabee a “very skilled player, a smart player who does a lot of things right.”

Farabee, 20, who had eight goals and 21 points in 52 regular-season games, had a strong Training Camp 2.0 but was beaten out of a lineup spot by Raffl (eight goals, 20 points in 58 games), a veteran who has also played up and down the lineup.

“He looks really good; he looks like he took a step forward,” Couturier said of Farabee, a former Boston University standout. “Just the way he battles and he’s involved with the puck and without the puck. He’s had a good year. It’s tough, your rookie year you have some ups and downs, and I think the important thing is to grow from it and learn from it. He came back [from the long pause] prepared, so we’re excited to have him in the lineup and we’re counting on him to make a difference down the road.”

Vigneault said he was still unsure about a couple of lineup decisions for Thursday. Among them: Does he give goalie Brian Elliott some work and sit Carter Hart, who was superb against the Bruins? And does he give defenseman Shayne Gostisbehere a start and rest Robert Hagg?

The coach wants to keep as many players sharp so they are ready if called upon, and must weigh that against fielding his best possible lineup in the round-robin seeding tourney.