Erik ten Hag refused to bring on Cristiano Ronaldo in the Manchester derby out of "respect for his big career".
Ronaldo was an unused substitute in United's 6-3 thumping at City as Anthony Martial came on ahead of him for his first appearance since August 22 and scored twice.
Martial's goals were his first for United in exactly a year and he is likely to start at Everton in their next Premier League fixture on Sunday. Ten Hag used all five substitutions available to him but Ronaldo stayed on the bench.
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"I wouldn't bring him in because out of respect for Cristiano, for his big career," Ten Hag said. "And the other thing was the advantage I could bring Anthony Martial, he needs the minutes but I don't want to point it out like that."
United lost 6-3 for only the sixth time in their history and the first time since the hammering at Southampton in October 1996.
Ten Hag had presided over four successive Premier League wins prior to the humbling at City, where United were 6-1 down at one stage in the second-half.
The United players were accused of being "undisciplined at following rules" by Ten Hag, who told the players what he told the press.
"I also told the players," he stressed. "Lack of belief. I have seen some highlights, it was obvious we are defending not on the front foot, we let them play, in possession we were not brave enough, made tactical mistakes, decision: mistakes and then you get hammered.
"They have to (accept criticism). They showed against Liverpool, Arsenal, when they perform as a team, when they bring the belief on the pitch, starting individual, we will show them and you cannot deny.
"In the last five games, we bring that attitude on the pitch, a strong belief, convincement, we bring opponents problems. Today, it was the other way around.
"It's normal in our process that pattern has to go up and I think I see clearly it goes up and we have setbacks during that process and we have to learn from it but point it clearly out.
"We can walk away from it but if we do it we won't get the solution. So we will do it, we will be very clear and we have to move on and carry on to the next game."
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