The Ghanaian midfielder produced an errant backpass to gift Jay Rodriguez the Saints only goal inside the first 15 seconds in Chelsea’s eventual 3-1 win at Stamford Bridge.
Despite the error, Mourinho did not criticise Essien too much even though he was forced to take him off at half-time after he picked up a yellow card just after the half-hour mark.
"Everybody makes a mistake," Mourinho told Sky Sports.
"Strikers they miss goals, goalkeepers they make mistakes, midfield players, they lose balls, especially when they try to play. And he tried to play. It was a back pass and they scored the goal.
"He had a very normal performance. He played a part in the stability the team had in the first half, but one yellow card for 45 minutes is a long time.
"I had to take one off and it was a very normal decision. Essien is Essien, he's my man, he's a Chelsea man and no problem, because other guys they make mistakes."
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