Henk ten Cate has already fired a warning to Chelsea players that a new sheriff is in town.
But Blues players were shocked when Ten Cate stopped a practice session this week because two players were laughing.
They were told off and warned that training was to be "serious".
A source close to the squad told The Sun: "If Ten Cate wanted to make a quick impression he has certainly succeeded.
"The lads were joking about in a way that would never have been considered disruptive in the past.
"But Ten Cate pulled them up and everyone was told there is to be no more laughing in his sessions.
"The players accept things will be different but they are quite surprised at what they have seen."