Hazard took his tally to five in five on Saturday as he scored a hat-trick in Chelsea's 4-1 win over Cardiff.
"I think [Hazard can win the Premier League Golden Boot]".
Chelsea's flawless winning start to the season is papering over the lack of firepower from their big money signing Alvaro Morata, and with Olivier Giroud hardly prolific, the big priority in the new year is likely to get another goalscorer through the door - a move for Mitrovic would be surprising but he indeed appears to be on the radar.
"We can improve.We are not so solid as I want in the defensive phase", Sarri said, citing Cardiff's early goal from an unmarked Sol Bamba.
Asked if Hazard believes Sarri's claim he can reach 40 goals, Sarri said: "If you have seen Saturday's match, then I think so!"
His best total came in 2014-15 when he scored 19 goals, and he scored 17 times in each of the past two seasons.More news: Reasons why Dybala to Manchester United rumours may be true
"In the moment we are very unsafe in the offensive phase but we are not solid as I want in the defensive phase", he added.
"I think Hazard can improve more", Sarri added.
"I would have thought other clubs might have put money down to take Hazard away from them - he's the one player that gives Chelsea potential championship [winning] ability".
"We have to improve step by step", said the Italian, who replaced Antonio Conte in the summer.
Chelsea are ahead of Liverpool on a goal difference of one, and two points in front of champions Manchester City. As a teenager, the Belgian had been a regular part of a good team, but upon turning 20 years of age he reached a level and has been on an nearly exclusively upward trajectory ever since.