Essien suffered a knee ligament injury in Sunday's record-breaking win over Birmingham and the Ghana midfielder is a doubt for Wednesday's trip to Reading.

After a pre-season scarred by injuries to a host of key players including captain John Terry, Mourinho must have thought things were looking up as Didier Drogba made a quicker than expected return to fitness against Steve Bruce's side.

Instead he could be facing a spell without his most versatile player.

Essien's value was underlined against Birmingham. He capped a dynamic display with the winner that ensured Chelsea extended their unbeaten run in league matches at Stamford Bridge to 64, beating Liverpool's previous top-flight record.

"It was something in the knee ligament," Mourinho said. "He (Essien) was limping and the doctor was saying he was in pain.

"I had two midfielders on the bench so I made the change immediately. If you ask me if he is ready for Wednesday I don't know.

"Claude Makelele will train with me on Monday, so will Paulo Ferreira. But Andriy Shevchenko, Terry, Wayne Bridge and Michael Ballack will be out."

Mourinho knows he cannot afford to take Reading lightly on Wednesday. Steve Coppell's side earned a gritty draw at Manchester United on Sunday and will fancy their chances of upsetting Chelsea.

As a result, Mourinho will resist the temptation to make significant changes to his team ahead of next Sunday's clash at Liverpool.

"These are different games, so maybe different choices but basically this is a moment when the players need to play," he said.

"I'm not going to make rotation like I would do if we were in February or March with Champions League matches around the corner.

"Reading played on Sunday like we did and it is the same for Liverpool next weekend. We play against Reading on Wednesday and they play Toulouse the same day."

Chelsea had to work hard for their record in a dramatic match. They fell behind to Mikael Forssell's early header, but Claudio Pizarro equalised two minutes later when goalkeeper Colin Doyle let his shot trickle in.

Florent Malouda finished a fine move to put Chelsea ahead before Olivier Kapo unleashed a stunning equaliser for the visitors.

Essien won it in the 50th minute when his curling drive was weakly palmed into the net by Doyle.

Mourinho is already setting his sights on reaching a personal milestone when Portsmouth visit on August 25.

If Chelsea avoid defeat in that match he will have got to a century of unbeaten home league matches during his time at the London club and Porto.

"I'm on 99 with my personal record and I hope my friend Harry Redknapp (the Portsmouth manager) does not kill my record of 100 in the next game at home because I would love to get there."

Birmingham boss Bruce took plenty of positives from the way his newly promoted team went toe to toe with Chelsea.

"If we keep producing performances like that we will make a fist of it that's sure," Bruce said. "We know it will be difficult but if we keep showing that attitude we've got an outstanding chance."

One source of optimism for Bruce was the performance of France midfielder Kapo.

Bruce said: "I thought he was arguably the best player on the pitch. I saw him playing for Auxerre against Arsenal four years ago and he was fantastic.

"He got a big money move to Juventus but it didn't quite work for him.

"He is a natural footballer and put in a good shift for us. He will enhance the Premiership and we are delighted we've got him."