Valerien Ismael said Barnsley lacked the 'killer mentality' and their defending was 'not good enough' in a 2-1 loss at Blackburn Rovers on Saturday.
The Reds created several chances but found themselves 2-0 down before Romal Palmer pulled one back late on.
Ismael said: "We have two faces. The first face says the performance was exactly what we wanted – aggression, intensity winning the ball deep and trying to finish quick.
"But the other one says we had 23 attempts, 11 corners and we scored once. We have to find the killer mentality. When you are in good shape in a game, you have to score because it is always the same, if you don't score the opponent will get one chance and score.
"We were very frustrated at half-time but we tried to stay focused. We were more close to the 1-1 than the 2-0 then after 2-0 we had to recover for five minutes. In the end it was 2-1 and it was mot enough.
"You have to analyse the opponent and the performance. We are good and stable, and we have the feeling that we improved our game. But I told the guys that we need the points and we need to punish the opponent by being killer on the pitch. We have to work on this."
On the defending for the goals, Ismael told the Chronicle: "It was not good enough. For the first one we have a clear principle for how we want to defend in behind but today we made a big mistake.
"But you have to accept this with young guys, next time they will do better. I am confident because it is our own mistake and, tomorrow, I can show the guys the clips to show why Blackburn scored."
Ismael was pleased with his side's attacking set pieces which led to chances. He said: "We worked on set pieces for this game yesterday. It is a good feeling to know we are very dangerous on the set pieces and we have to work on that.
"Our set pieces were almost perfect. We have to score, this is the Championship. We can build from this performance for sure because we have consistency in our performance, but you need a reward."
Herbie Kane made his first start for the club today. He said: "It's massively frustrating to lose, with how hard we worked and the chances we created. But there was no conviction. It's disappointing but we have a game on Tuesday and hopefully then we'll be more clinical .
"I thought we dominated throughout the game, we had a load of chances. They took their chances and that's what wins you games. Personally I thought I did well but it doesn't really matter because we lost."
Ismael added on Kane: "He did very well. I knew Herbie was very near to the starting 11 and every time he comes in he does very well. He did his job perfectly. I am very happy for him."
Palmer got the Barnsley goal, and his first in senior football, after coming off the bench and playing more of an attacking role than usual. Ismael said: "I am very happy for him. I spoke with him this week. I told him to be patient and his time is coming. We need players who can come on and play well. If you stay calm and you are patient and work hard, you can help the team.
"I told Romal we have to take the risks to play more high. I told him Herbie is more the defensive part and he has to push forward."