Michael Raffl, Connor Bunnaman and Morgan Frost are among the options to replace Couturier if he is unable to play, and Giroux could be shifted from left wing to center.
Raffl played Games 1 and 2 of this series but has been scratched since.
"Nobody can replace (Couturier). (He's) is one if the best two-way centers in the league, plays that 200-foot game, very dependable and trustworthy," Vigneault said. "We'll see what unfolds here tonight. There's no doubt that whoever is in the lineup, guys have to play well because we definitely have to be at our best tonight."
Trotz gave his players the day off Wednesday after they rallied with two goals in 1:33 late in the third period to force overtime. The Islanders and Flyers will each play their 15th game of the postseason Thursday.
"Today, we didn't do anything," Trotz said. "We're going to take a little bit of a mental break from this. We've been sort of going at this for about two months now, and people don't realize we've been in the bubble for about 5-6 weeks here, but it started beforehand. You're pushing the guys to get to a place mentally so they could have some success, and we've got through two series already. We'll get them energized, and they'll be ready tomorrow. There's no question. I don't have any fear. I'm actually enjoying today in a sense that we're getting some work done, going to have some conversations and we're going to be ready to go tomorrow.
"This is where you find out everything about each individual player, if they're willing to go that extra mile. We've got to go that extra mile, just as every player at this time of the year, in this format, is doing."
NHL.com deputy managing editor Adam Kimelman contributed to this report