The NHL's department of player safety announced the punishment in a release.
The incident took place at 18:09 of the first period of the Capitals' 3-2 win, when Simmonds and Erskine were both going for the puck in the Flyers' zone. The latter got his elbow up high on the former, leaving Philadelphia's leading scorer sprawled out on the ice with a bloody nose and forcing him out of the game.
No penalty was called on the play, but Flyers general manager Paul Holmgren told reporters the forward sustained a concussion.
"As the two men approach the puck, Erskine looks up and clearly is aware of Simmonds' proximity before delivering the illegal hit," NHL discipline czar Brendan Shanahan described in a video explanation. "While we acknowledge there was no malicious intent and Erskine was reacting to getting beat, it was an elbow to the head that caused a serious injury."
Shanahan also noted Erskine isn't a repeat offender.
As a result of the suspension, Erskine will forfeit $24,324.33 US with the money going to the Players' Emergency Assistance Fund. He'll miss two games against the Pittsburgh Penguins, and one against the Toronto Maple Leafs before being eligible to return Feb. 9 against the Florida Panthers.