If anyone is interested, click the Twitter link below for video of the Couturier injury. I'm not an expert (Andy Sutton says so), but, if I had to make a guess, I'd say he hyper-extended his knee, and maybe stretched some ligaments at worst. The way Gudas hit him looked more like the knee bent backwards than twisted, and twisting is usually where you see the more serious injuries. Yes he was limping, but he also was able to put some weight on it.
I wouldn't be totally shocked if he did play tomorrow, being the playoffs. Regular season, he probably sits out a few games.
Here's the link: (Nice stick toss, BTW)
https://twitter.com/JamieApody/status/986318648313774081