I can agree with you that Erik Ten Hag took this game seriously and played his best player available but his team suffered a bad day, conceding a goal in the 92nd minute of the game, I bet no one saw that goal and United players were confident the game was concluded and won already. Good game for Crystal Palace I'm pretty sure they will be happy with the result from the game.
Erik Ten Hak has always taken the game seriously in the Premier League, even though he has to face teams at the bottom of the table, he only rotates one, two and three players when deemed necessary and that is why Manchester United is slowly improving and Erik Ten Hak has managed to bring Manchester United to third place big standings. Michael Oise's goal in the 91st minute cost Manchester United a place in second place in the standings, three points adrift of Manchester City, who beat Tottenham 4-2.
Crystal Palace deserved a draw, because they played quite well in that match, a draw in this match also made Crystal Palace occupy 12th place in the standings, five points adrift of Chelsea who are currently in 10th place.
I understand that MU drew against Crystal Palace. It is possible that MU players are tired both physically and mentally because the match schedule is so tight that it makes their performance less than optimal.
Apart from MU failing to kill the game with many chances, Palace often makes it difficult for the big teams, especially at home. Especially in the EPL the bottom team can sometimes beat the top. A draw is too expensive because Casemiro got a yellow card. next MU Vs Arsenal without casemiro
Erik Ten Hak has to take this risk, even though he is aware of the impact fatigue has on the players he has, because Manchester United's substitutes are not very good and they don't have the same quality as their core players, so Erik Ten Hak has no other choice in rotation. Erik Ten Hak managed to overcome this and the Red Devils are slowly getting back on track for the title race this season.
Crystal Palace always surprises the top teams and in that match they played quite well in defense and attack, so that Manchester United had difficulty scoring goals, without the presence of Casemiro Erik Ten Hak had to think of another strategy against Arsenal.