Hertl, who has scored seven goals in his past eight games, left the game favoring his right arm at 14:11 of the third period after being boarded by Emil Lilleberg.
“He's getting evaluated now and again tomorrow,” Golden Knights coach Bruce Cassidy said. “He was sore when he came off, so we'll see. We'll maybe get good news tomorrow.”
Nick Paul and Nikita Kucherov scored, and Andrei Vasilevskiy made 26 saves for the Lightning (40-25-5), who lost 6-4 at the Utah Hockey Club on Saturday.
Tampa Bay is third in the Atlantic, two points behind the Toronto Maple Leafs.
“We had a lot of great looks,” Lightning forward Brandon Hagel said. “We believe we're a really good hockey team over here. We have 12 games. That's kind of the message in here. Throw out the guys and let's start building something here. It's not just a light switch that turns on 12 games from now. It's got to start now. I like that for tonight. Obviously, can we get better? Absolutely. But we've just got to continue to build on it. It can't be one good, one bad, one good, one bad. We need to stay together.”
Roy made it 1-0 at 11:02 of the first period, scoring a power-play goal with a shot that went in off the crossbar.
Hertl made it 2-0 from the high slot at 13:33, and Eichel one-timed a Zach Whitecloud pass at 19:13 to extend it to 3-0.
“Him and I actually work on that in practice just to fool around and stuff,” Whitecloud said. “He's been working with me to help get a little bit better vision in the offensive zone and stuff like that, so when you see [Eichel] with open space like that, you try and get him to puck, so he did the rest.”