Giovanni van Bronckhorst says Feyenoord were guilty of making too many mistakes in their 3-1 loss to Napoli.
Lorenzo Insigne opened the scoring in their Champions League clash at the Stadio San Paolo after just seven minutes before Dries Mertens' 49th minute goal ahead of a third from José Callejón with 70 minutes played.
Sofyan Amrabat netted for Feyenoord in the third minute of stoppage time at the end of the game.
Jens Toornstra was also guilty of missing a 68th minute penalty for the Eredivisie side, who had a number of key players missing.
After the game, a deflated Van Bronckhorst said: "We conceded a goal straight away, but we remained in the game until half-time.
"After that, we made two mistakes and missed a penalty.
"In the Champions League, it's all about moments and we made too many individual errors.
"Admittedly, it is difficult at the San Paolo. We expected Napoli to play like this. After the goal, we tried to stay solid, but we had some unlucky situations and errors.
"Napoli play football very easily and they are talented. Unfortunately, with so many of our players missing, it made a big difference."