Chelsea have a great chance to record a fifth straight win when they hose Cardiff City this weekend but even if they do have 15 points from the first five games, it looks like Sarri will not be totally convinced of his side's title chances.
David Silva doubled the lead in the 21st minute, scoring his 50th league goal in his 350th league game when he finished past Bettinelli following a deflect, and Raheem Sterling scored his third goal of the season in the 47th minute on a cross from Sergio Aguero after Ryan Sessegnon lost possession.
Hazard completed his hat-trick from the penalty spot before Willian's stunning strike completed the rout seven minutes from time.
The Belgium worldwide scored his third with a late penalty after Bamba brought down Willian, who added the Blues' fourth from long range.
Into this turbulent environment has walked Maurizio Sarri, dressed on the touchline in a scruffy T-shirt and cap, looking less like the boss of a Premier League club and more like a bloke who has come to paint the kitchen.
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"I think that, if John prefers to start with the young players, it's right to start with the young players".
Chelsea are forever involved in some soap opera or another while continuity seems to be a dirty word at Stamford Bridge.
Chelsea and Cardiff last played one another in a Premier League fixture in the 2013-14 season.
"I think Hazard can improve more", Sarri added. "But now I change my mind because maybe he is the best", said Sarri of the man crowned the second-best player at the World Cup this summer.
United is getting its season back on track after a 3-0 humbling at home by Tottenham, with victory before the global break at Burnley followed by the success at Watford.
"In the second half, maybe our ambition went a little bit down", Mourinho said. The midfielder should certainly have converted the second, a one-on-one with Etheridge, and his hard afternoon came to a frustrating end five minutes into the second half as he limped off with an apparent muscle problem in his right leg.