Gernot Rohr said defensive naivety from set-pieces was to blame as Nigeria's World Cup opener ended in a 2-0 loss to Croatia.
Oghenekaro Etebo's own goal from a Luka Modric corner gave Croatia a first-half lead, with the Real Madrid midfielder making sure his side top Group D by converting a spot-kick after the interval.
William Troost-Ekong needlessly brought down Mario Mandzukic while defending another corner to hand Croatia a penalty that wrapped up the game.
Nigeria's lack of creativity was evident, but Croatia did not record a shot on target from open play until second-half stoppage-time, leaving Rohr in no doubt what went wrong for Nigeria in Kaliningrad.
"We are disappointed because we don't like to lose but we must recognise Croatia had the better team," the Nigeria coach told a post-match media conference.
"My young team made some mistakes from corners, we worked from them so much but both goals came from corners, which I find a little bit… There were also so many fouls in the other box that could have been penalties but we have to accept the defeat.
"Of course you are not satisfied with the attack when you do not score, we were satisfied with our defensive organisation but we didn't have so many opportunities to score.
"I think the organisation was OK, we didn't concede in open play. We conceded twice in set-pieces so it is not a question of tactics or system.
"I was satisfied with the organisation but we need to be more professional from set-pieces."
Nigeria face Group D giants Argentina in their final fixture, Lionel Messi having missed a penalty in a 1-1 draw against World Cup debutants in Moscow earlier on Saturday.
And although Nigeria's task now looks tough Rohr is not giving up hope of leading his side into the knockout stages of the tournament, with Iceland up next for the Super Eagles.
"It's important to make at least four points if you want to qualify," Rohr added.
"We are only a point behind for the moment, nothing is lost and all is in our hands.
"Let's be positive, we have the youngest team at the World Cup, let them learn and try to do better.
"We saw Iceland have a good game against Argentina, a very strong team, but we can also do better than what we did today.
"We have to win against Iceland but all is possible still. If we win the next game all is possible for qualification."