The defending Serie A champions had to settle for a point after playing to a 1-1 draw with Lecce on Saturday afternoon.
The result did not affect their top spot in the standings as Inter Milan were unable to get past Parma.
However, Sarri admitted that Ronaldo was not 100 percent for the trip to Lecce on Saturday.
"Cristiano Ronaldo was tired both physically and mentally, so it wouldn't have helped him to make this journey and sit there. When a player expresses his fatigue, it was only right to give him a rest," Sarri was quoted as saying by Football Espana.
"The fact remains, we created 10 scoring opportunities and didn't finish them. I think we took it a bit easy in our finishing and after we took the lead, acting almost as if it was natural we'd win.
"I don't know if we were superficial as such, but we weren't as determined as we could've been. We shouldn't think that just because we create lots of chances, we'll automatically score and win."