Erik ten Hag provided a lengthy reason behind substituting Rasmus Hojlund Kobbie Mainoo around the 65th minute of the 1-1 draw to Burnley.
Defending his decision after receiving loud boos, the Manchester United boss told reporters, quoted by Goal: "What the crowd doesn't understand is it's a 19-year-old boy [Mainoo] in his first Premier League season.
"It was a third game in a week and we have to manage the game load. We don't want him to be injured. Last year, he had two bad injuries. You can see he gets tired. I want to protect him.
"Of course, by bringing Scott [McTominay] on, the game was really open, really intense with one-on-ones all over the pitch.
"Kobbie got tired and Scott had fresh legs and is very good, making movements and attacks down the middle."
He continued: "I have to manage the team and I have to, of course, be winning games.
"That's first of all and see what the game needs. By that time, we needed some freshness, first of all. But also tactical wise, brining a player up who can keep the ball and who is creative in Amad."
Ten Hag insisted bringing on McTominay and Amad was correct: "It was the right decision. You see, after, we took some benefit from it, by bringing in the fresh legs and Scott in the position where it's so open.
"It's totally logical to bring players off who are very young, who are in the first year in the Premier League and then don't even talk about the injury risk.
"Rasmus Hojlund had more injuries already in the course of this season. Kobbie also had some injuries. So I don't want them to get injured. I protected them as well.
"I understand that the fans, they want to see the skills those players are bringing and contribute to the team. I understand that but I have to do my job and that is the right decision."
Hojlund barely received any service at all and Mainoo was one of the few bright sparks in the otherwise drab encounter.
Bringing them off led to Antony's goal but that meant nothing after Andre Onana conceded an unnecessary penalty, allowing Burnley to equalise.
