Juric admits he expected more from his changes as Saints were beaten by a late hat-trick struck by Amad Diallo.
"I have to be honest, I expected a little bit more of everybody when they came in. They have to give us more energy, more intensity," said Juric.
"As a team, we did a really good game for 70 to 75 minutes, everything was almost perfect - but we had to score more goals.
"But there is a moment you have to be a little bit more evil, I can say, manage the game differently and we didn't do that," Juric added.
"We had really bad managing the game. You cannot play 95 minutes at the same level. There is a moment when you have to manage things differently.
"You have to win the throw-ins, win the fall. We lost some balls in the middle and I didn't like it. It's really a pity, it's a shame that we play like this and lose."
The result leaves Saints bottom of the Premier League table.
Juric added: "It has to be like a positive thing for us that they can play like this against Manchester United.
"If they are competitive here they can every game. I think the guys understand we have to work even harder to get fitter and we will see in the next game."