The Blues had choked their lead against The Monegasques twice before the game ended in a 2-2 draw, and the 25-year-old striker has now conceeded Manchester City needed to have done more.
Speaking with TNT Sports, he said: “We don't feel good. We don't win, we do something unnecessary in the second half, and I don't think we played well enough, so we don't deserve to win. That's why.
“I think we need more energy, we need to get at them more as we did in the first half. We dominated much more, and now in the second half, they took the lead much more. I don't think it's good enough.”
In the game, Monaco’s Eric Dier, 31, scored a 90th-minute penalty - which was given after a VAR check for a foul by Nico Gonzalez, 23 - to earn a 2-2 draw and cancel out Haaland’s two first-half goals at Stade Louis-II.
Manchester City had 18 shots and over 70 per cent of the posession, though couldn’t translate their dominant performance into a winning result.
Haaland scored twice in the first half from only seven touches – first with a casual flick, then with a powerful header – showing not just his efficiency on the ball, but also the pressure his presence puts on defenders.
He added: “I still think I'm involved in the game, doing movements and giving space to others, so it's not only about touching the ball if you're involved in the game or not.
“I think you can be involved in the game many other ways, and that's my job. I did my job in the first half, in the second half I didn't, so yeah.”
When pressed on whether he felt he had done his job in the second half, Haaland said: “Of course not, I didn't score, I didn't finish the game, so no.
“But every Champions League game is tough. Look at last year, we went out, so every single game is tough. There aren't many teams that have won their first two games, and that's how it is.”