At 11:06 of the first period, Neal struck Marchand when the latter was on the ice and was assessed a minor penalty.
Bruins forwards Loui Eriksson and Chris Kelly were also injured in the Pittsburgh game.
Eriksson suffered a concussion after being hit by Penguins defenceman Brooks Orpik and did not travel to Toronto for the start of the Bruins' four-game road trip on Sunday.
Boston placed Kelly on long-term injured reserve after breaking his right fibula against Pittsburgh.
Bruins tough guy Shawn Thornton will have a in-person disciplinary hearing with the NHL for punching Orpik and sending him to hospital.
Click below to listen to NHL disciplinarian Brendan Shanahan's decision for Neal's suspension.