The Devils announced his suspension the day after the game with the Islanders.
Mottau came across the ice 3:22 into the third period and caught Nielsen with an elbow that laid out the centre. Nielsen had to be helped off the ice with head and leg injuries.
Trent Hunter immediately came to Nielsen's defence and fought Mottau, earning an instigator penalty and a misconduct in addition to the fighting major. Those penalties wiped out the power play the Islanders would have had for Mottau's initial hit, which only drew a minor penalty for elbowing.
"I know it was to the head," Guerin said. "I know Mike Mottau. He's not a dirty player, but it was a dirty hit. Thomas Pock is not a dirty player, but he had a dirty hit against Ottawa and he paid for it."