The England international was dismissed just over midway through the first-half, leaving his team-mates to play out the majority of the game with a numerical disadvantage.
Smalling’s first yellow card was needless one, as referee Michael Oliver dished out a caution after the defender blocked a Joe Hart goal kick.
The United man then lunged in on City’s James Milner to pick up his second card and seal his dismissal.
After the game Van Gaal said he has no problem with the decision to send off his star, and that the player himself had acted in a stupid manner:
“I think the first yellow card for Chris Smalling, I did not see but OK, the referee gives a yellow card,” he told Sky Sports. “Then you know as a player you have a yellow and you cannot do what he did for the second yellow card. That is logical.
“I say also a lot of things to the players and despite that they do not always follow what I have said. There is also emotion and sometimes you cannot control your emotion but it was not very smart.”