The Anaheim Ducks defenceman received a one-game suspension Sunday after hitting Ottawa's Dean McAmmond in the head with his forearm during Game 3.
"This one took a lot of thought," Colin Campbell, the NHL's executive vice-president and director of hockey operations, said at a news conference. "It wasn't an easy one. It didn't jump out at us right away.
McAmmond left Saturday's game with a head injury and is doubtful for Game 4 on Monday.
"McAmmond doesn't look promising," Senators coach Bryan Murray said.
Anaheim leads the NHL championship series 2-1.
"We accept the league's decision," said Ducks coach Randy Carlyle. "We feel that the player did not attempt to hurt Dean McAmmond.
"We have to live by the league's decision."
It is the sixth career NHL suspension for the six-foot-six, 220-pound, hard-hitting all-star and the second in these playoffs. During the previous round against Detroit, Pronger was banned for Game 4 after his hit to the head of Red Wings forward Tomas Holmstrom against the boards.
Pronger wasn't penalized on the ice for either of the hits that led to his suspensions.
McAmmond was hit as he skated with the puck toward the Ducks net. He did a spin, fell backward, struck his head on the ice, and slid into the corner in the Anaheim zone at 2:01 of the third period.
He briefly lost consciousness, Murray said, but there was no official announcement that the veteran of 14 NHL seasons sustained a concussion.
After receiving medical attention for several minutes, a dazed and woozy McAmmond was helped to the dressing room and didn't return.