The Stars forward sustained the injury at 2:05 of the second period in a 5-4 win against the New Jersey Devils on Wednesday but is expected to return at the Winnipeg Jets on Sunday. Benn, who played in all 82 games in three of the past four seasons, will miss his first game since March 17, 2017.
Dallas is 5-2-1 in its past eight games.
The Capitals, who lost 5-2 at the St. Louis Blues on Thursday, will try to avoid losing three straight games for the first time this season. Washington has been outscored 11-5 in consecutive defeats following a four-game winning streak.