Benn was tossed in the first period of Tuesday's 4-0 drubbing in Dallas after striking Knights forward Mark Stone in the neck area after taking him down in the penalty box.
NHL disciplinary chiefs said the incident demanded further sanction given that Stone was on the ice when he was struck.
"It's important to note that Benn is in control of this play at all times and makes the decision to deliver a forceful cross-check to a prone player," the NHL said.
"This is simply an unnecessary decision by Benn and it is delivered with sufficient intent and force to merit supplemental discipline."
Golden Knights goalscorer Jonathan Marchessault later branded Benn's challenge "disgusting" following Tuesday's game.
"It's just not a hockey play right?" Marchessault said. "It was pretty disgusting - he leans on top of him and gives it to him in his head."
The Stars are 3-0 down in the best-of-seven Western Conference finals and facing elimination from the Stanley Cup play-offs.
Benn later described the incident as "unfortunate."
"I think I just need to be more responsible with my body and my stick and put my team in a tough situation," Benn said. "It was pretty unfortunate."
