The incident took place with 2:13 left in Sunday's 3-1 loss to the Wild. Reacting to a check on linemate Pavol Demitra, Tkachuk cross-checked an unsuspecting Walz in the neck, drawing a match penalty for intent to injure.
"Blows with the stick to the head area continue to be unacceptable," said Colin Campbell, the NHL's director of hockey operations.
Match penalties automatically warrant review from the league, although they do not come with mandatory suspensions.
Tkachuk defended the hit, claiming he connected with Walz's shoulder.
"If I wanted to (hit him in the face), I could have," said Tkachuk, who leads the Blues with 28 goals. "I would never attempt to hurt anybody, I don't care who it was."
Walz was not hurt or angered over the check.
"It's part of the game," he said. "If he wanted to hit me in the head, he could have. He didn't get me in the face or it could have been worse. I think it looked worse than it was."
As a repeat offender, Tkachuk forfeits $487,000 of his salary.