But Napoli and Mertens could not force an equaliser and coach Sarri explained his decision, insisting it could have paid dividends had Facundo Ferreyra not nodded in a second for Shakhtar.
"[On Tuesday] I was accused of not making enough changes; now I am accused of too many," he told Mediaset Premium.
"Dries is extraordinary and we don't leave him out lightly, but pace is his greatest strength and he can't have 50 to 55 games per year.
"He was coming off international duty, too, so we decided to rest him and let him come on later.
4 - Napoli have lost four of their last seven CL games (excl. qualifiers), winning just once in the process. Hard. #ShakhtarNapoli— OptaPaolo (@OptaPaolo) September 13, 2017
"If we had managed to keep it at 1-0, he could have got us the equaliser. He has certain characteristics and qualities so it's not easy to leave him out, but sometimes we have to."
"We must not forget that we lost 2-1 at the home of the seeded team in this group," he added.
"If anyone thought the Champions League would be a walk in the park for Napoli, they have their feet back on the ground now."
Despite a disappointing Napoli display, Shakhtar coach Paulo Fonseca felt his side deserved credit for their own performance.
"To Napoli, I say that it is not fair to say that they took the wrong approach to the game," he told a post-match news conference.
"From my point of view, we were good at cancelling out their strengths. We won against a very, very competitive opponent - and it was on merit."