The 31-year-old battled illness in the lead-up to Sunday's trip to Goodison Park and made way for Olivier Giroud after an ineffectual 66 minutes.
His fluffed close-range finish early in proceedings typified Chelsea's wasteful start and the visitors paid the price through second-half goals from Richarlison and Gylfi Sigurdsson.
Higuain, who joined on loan in January, has now scored just once since a double against Huddersfield Town on February 2, raising questions over his long-term future in London.
"In this moment, yes of course [he is struggling to score]," Sarri said.
"Today he had an opportunity. Too few for him I think. He can do more.
"He was improving, but then he had some problems at the beginning of the week. He had a strong stomach fever so today it was normal that he could only play for 60 or 65 minutes.
"He needs to improve his physical condition and mental condition."
Chelsea's first defeat since the Carabao Cup final came as the result of a listless second half and left them three points adrift of the Premier League's top four.
"It is not impossible. Why impossible? We need to recover only three points I think so it is not impossible in eight matches," he added.
"It is impossible if we cannot improve and not able to avoid a second half like today.
"We need only to play like the first half for eight matches, without stopping playing and stopping to defend. This is the limit."