The incident occurred in the third period of Saturday's game.
Seabrook incurred a five-minute charging penalty on the play, and Backes was injured.
The NHL Department of Safety ruled that that the hit was late and to the head, qualifying as both charging and interference as Backes did not have the puck. Seabrook also erred by driving upward into the check.
Vladimir Tarasenko of the Blues tied the game on the ensuing power play, with 64 seconds left in the third, and St. Louis went on to win in overtime to take a 2-0 series lead.
Game 3 will take place in Chicago on Monday, and will be shown on Hockey Night in Canada (CBC, CBCSports.ca, 8:30 p.m.ET).
Seabrook will be eligible to return on Sunday April 27 for Game 6, if necessary.
It is not yet known how long Backes will be out of the lineup. The Minnesota native scored 27 goals and added 30 assists for the Blues this season.
Here is the hit, and the NHL explanation of the suspension: