Oshie suffered the injury after Carolina Hurricanes forward Warren Foegele hit him from behind Thursday.
The incident occurred with just over five minutes remaining in Game 4 when Foegele hit Oshie from behind, sending him tumbling into the boards at an awkward angle. The 32-year-old didn't return to the game, and Foegele was assessed a two-minute minor for boarding. The rookie didn't receive further discipline.
Oshie was a key contributor throughout the Capitals' Stanley Cup run last spring, recording eight goals and 21 points in 24 contests. He notched one goal and one assist in four games against the Hurricanes this postseason before the injury.