Manchester United manager Louis van Gaal has labelled Chris Smalling's sending off "stupid", after the defender was dismissed mid-way through the first half in his side's 1-0 defeat to Manchester City.
Smalling was sent off for a second bookable offence, picking up a yellow card for obstructing Joe Hart's drop kick, and another for a late sliding challenge on James Milner. van Gaal's side battled bravely with a makeshift back four, which consisted of Valencia, Carrick, McNair and Shaw after Smalling's departure and an injury to Marcos Rojo.
But United were eventually broken down in the 63rd minute, when Sergio Aguero fired in from a Gael Clichy cross.
The visitors pressed for an equaliser, but with 10 men, City proved to be too strong.
"In a derby you have to be careful - the second yellow card is a stupid yellow card," van Gaal said.
"The sending off is not one of those things. As a player you have to control your aggression.
"I didn't see the first yellow but with the second, you know you already have a yellow, so have to handle it differently. I said that to the players."
Smalling's red card will mean he misses the upcoming fixture with Crystal Palace, compounding United's problems at the back. Marcos Rojo is also likely to be sidelined, where he will be joined by Phil Jones and Jonny Evans.
"I have seen good performances from Michael Carrick and Paddy McNair, but it's a disaster for us," Van Gaal added .
"So many defenders are injured and you have to play each week with another defence."