The Belgium international had to be replaced for the final 16 minutes at Stadio Olimpico after grabbing his ankle, but Sarri thinks the problem was only muscle cramps, although he will not know for sure until the players have been assessed.
"Mertens is fine - I think there was only a suggestion of cramps, but we will assess him better [before the game] along with the rest of the squad," the Napoli head coach said at his pre-match media conference.
Sarri is unhappy with the busy run of fixtures his side have had around a crucial Champions League last-16 tie, which they trail 3-1 from the away leg.
He continued: "I don't want controversy, but the calendar certainly has not helped us. I have never seen so many important matches pushed together.
"Certainly, from this point of view, it could be better. You know who makes the Champions League and some things were not taken into account in the calendar to protect Italian teams in Europe."