The NHL's department of player safety made the announcement after having a hearing with Pionk on Monday.
The incident occurred with 14:15 remaining in the third period Sunday when Winnipeg hosted Toronto. As Sandin received a pass in the Jets' zone and proceeded to take a shot on net, Pionk attempted to line up a body check, but misjudged Sandin's position, causing him to extend his leg and create knee-on-knee contact with the Maple Leafs defenceman.
"We acknowledge Pionk's argument that this is an attempted full-body check, but this play cannot be classified as merely a collision between two players, where one or both move reflexively or defensively at the last moment to avoid contact," said the department of player safety in a video explaining the punishment.
"If he wants to deliver this hit, the onus is on Pionk to take an angle of approach that ensures he's in good position to make a legal, full-body check. Instead, having taken an angle which has him lined up outside of Sandin's path, Pionk turns his right leg and extends his knee, jutting it forward to avoid missing the check entirely. This results in forceful, dangerous and direct knee-on-knee contact."
Watch the full video breakdown here:
Sandin was helped off the ice and appeared as though he could not put weight on his right leg after the play. Maple Leafs head coach Sheldon Keefe said he had no update on Sandin's condition on Sunday night.
Pionk, who has never been suspended or fined before this event, will forfeit $58,750 in salary.
Just two minutes after the incident, Maple Leafs forward Jason Spezza retaliated with a knee of his own -- targetting Pionk and catching him near the head as he slid on the ice to make a play.
Spezza is at risk of also receiving a suspension and will have a hearing with the department of player safety via Zoom on Tuesday afternoon.