"We were competitive for a long period of the game and were not inferior to Napoli but the outcome of the game was decided by personal mistakes," the former Netherlands international told reporters.
"They were expensive errors on our part. Under pressure you have to make quick choices and ours were not good," he added after Feyenoord suffered a second Group F loss following their 4-0 home thrashing by Manchester City earlier this month.
Defensive mistakes led to all three Napoli goals and Jens Toornstra also missed a penalty when the Dutch champions were 2-0 down with a little more than 20 minutes to play.
"The penalty could have been a turning point and it was a real pity it didn't go in. Those were among the costly errors," he added.
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Captain Karim El Ahmadi said mistakes at Champions League level were quickly punished. "We were too naive at times but we can't dwell on that too long, we must move on," the Moroccan midfielder said.
Feyenoord did get a consolation goal with the last kick of the game, their first in the group phase since Anthony Lurling scored in a 2-3 home defeat by Newcastle United in 2002.
The Dutch champions next play Ukraine's Shakhtar Donetsk at home on Oct. 17.
(Reporting by Mark Gleeson; Editing by John O'Brien)
