Maurizio Sarri believes his Napoli side were "negatively affected" by Benfica's draw at Besiktas as they also slipped up and were held by Dynamo Kiev in the Champions League.
The Serie A side would have progressed to the knockout stages had they and Group B rivals Benfica both won on Wednesday, but the Portuguese outfit blew a three-goal lead to draw 3-3 before the later game kicked off at San Paolo.
And after seeing his Napoli team booed by their supporters at the end of a goalless stalemate, Sarri admitted that the group's other result was to blame as no result could guarantee qualification.
"We went out for the warm-up with Benfica winning 3-1, and then the news came that it was 3-3," he told Mediaset Premium.
"The boys realised that a win or a draw would not give us anything more. This has negatively affected us.
"Up to a quarter of an hour from the beginning [of the match], there was great desire to win. It is difficult not to be influenced by anything."
Benfica top Group B ahead of their final game against Napoli, who are level on points with eight apiece.