Essien scored the winner in the match against Birmingham
The Ghana star was substituted with 21 minutes remaining of Sunday's 3-2 win.
"It was something in the knee ligament. He was in pain, I had two midfielders on the bench so I made the change immediately," said Mourinho.
"If you asked me, I would say I do not know if he will be fit for Wednesday's match against Reading."
Mourinho enjoyed some positive news as Didier Drogba came off the bench after missing the Community Shield match against Manchester United because of injury, while Claude Makelele and Paulo Ferreira are expected to train on Monday.