The Manchester United defender walked after a bad challenge on James Milner saw him earn a second yellow card.
Sergio Aguero's strike in the 63rd minute gave Manchester City a hard fought 1-0 win over Manchester United in the derby at the Etihad Stadium, but Louis van Gaal felt that Chris Smalling's sending off made all the difference to his team.
Smalling was sent off for two bookable offences in the first half, the second a nasty challenge on James Milner.
“In the derby you have to be careful. The second yellow card was stupid,” he told BBC Sport.
“As a player you have to control your aggression. I didn’t see the first yellow, but the second you know as a player you have a yellow, so you have to handle it differently. I said that to the players.”
Van Gaal revealed that Smalling apologised after the game, while his fellow defender Marcos Rojo will not play against Crystal Palace next weekend after dislocating his shoulder.
