Saints staged a second-half fight-back, but fell short to lose 3-2 at City Ground.
"I think this team, when we play without intensity, we are really bad. We didn't have intensity," Juric admitted.
"Our energy for the first two goals was really bad. We are really disappointed after a big match against Manchester United that we played the first half at this level.
"In the second half, I think even with the guys who came in, we had much more energy. We changed, we scored two goals, we attacked.
"We had to score one more but it was the team that I wanted to watch. Maybe we are not ready to play two games in 60 hours on a high level.
"Maybe. But then again, in the second half, we pushed a lot. I had fear for (Kamaldeen) Sulemana to start from the beginning, Lesley, and they gave us lots of energy."
Juric added: "Sulemana doesn't play for 90 minutes for two years and he had lots of injuries and we had to be careful about him.
"Lesley (Ugochukwu) again, young players are the players I liked today, with energy, strong, knows how to play and to fight, everything.
"I wanted to give the minutes to Flynn Downes to come back I think he's a good player - but he didn't do well. I think it's a big difference in the Premier League.
"When I saw against Swansea, there were two levels. Championship and Premier League. If you're good in the Championship, it doesn't mean you're good in the Premier League.
"But I think Flynn is good. Flynn can play in the Premier League really good. He did it against West Ham and against Fulham he was really good.
"Now, after the injury, he's maybe a little bit down, but I have to recover him and he will be good."