Maurizio Sarri said he cannot rotate Lorenzo Insigne as the "world class player" is too important for Napoli following their Champions League victory.
Insigne scored a stunning goal to lead Napoli to a 3-0 win at home to Shakhtar Donetsk in Group F action on Tuesday.
Deemed surplus to requirements in Italy's failed World Cup qualification bid against Sweden, Insigne struck against AC Milan in Serie A last week and the star forward was at it again as Napoli accounted for Shakhtar.
"This is Lorenzo. He has improved so much over the last three years, become a world class player who I think has to be left out of squad rotation too, because he is so important," Sarri told Mediaset Premium.
"We can give him a rest for the final 20 minutes, but to drop him from the start – I can't do it."
The win boosted Napoli's hopes of progressing to the knockout round in the Champions League.
Napoli are still third in Group F, but they only trail second-placed Shakhtar by three points with one matchday remaining as Manchester City sit six points clear.
"We wanted the three points at all costs," Insigne said. "We know it doesn't only depend on us, but we hope City will beat Shakhtar and we can get our result away to Feyenoord."
On Sarri, Insigne – who was dropped by former Italy head coach Gian Piero Ventura for the World Cup play-off – added: "I thank the coach, as since the first day he has always put his faith in me and I tried to repay him with performances, goals and assists.
"I'll never stop putting myself at the disposal of the coach at any moment of the match."