LONDON, Dec 26 - Chelsea's defensive frailties cost two valuable points in their quest for a hat-trick of Premier League titles on Tuesday when a Michael Essien own goal gave Reading a 2-2 draw at Stamford Bridge.

Leaders Manchester United took full advantage later in the day, extending the gap at the top to four points by cruising past Wigan Athletic 3-1 at Old Trafford with Cristiano Ronaldo continuing his hot streak with two more goals.

United have 50 points from 20 games while Chelsea have 46. Arsenal, who beat Watford 2-1, replaced Liverpool in third place with 36 points, the same total as Bolton Wanderers who edged Newcastle United 2-1.

Liverpool dropped to sixth as their remote title hopes disappeared with a 1-0 defeat at Blackburn Rovers which left them 16 points behind United.

Benni McCarthy struck for Blackburn, who had not beaten Liverpool in 17 attempts.

In the absence of influential captain John Terry, Chelsea have conceded six goals in three league matches and manager Jose Mourinho says he does not know how long the defender will be missing.

"I have no idea when John Terry will be back, nobody has told me if he is going for back surgery, if it will be three days, three weeks or three months," said Mourinho, who was also without midfielders Joe Cole and Arjen Robben.

"The big problem is when you lose more than one crucial player...to lose (goalkeeper) Petr (Cech) and John is a big problem for us. At this moment we are conceding too many goals.

"Normally Chelsea in three consecutive matches have three clean sheets but in the last three we have let in six goals," Mourinho told reporters.

When Reading and Chelsea met in October the game was marred by a serious head injury to Cech.

Didier Drogba's 11th and 12th league goals of the season seemed to have sealed victory for Chelsea on Tuesday.

STOOPING HEADER

But Reading, who had equalised after 67 minutes through Leroy Lita's stooping header, got a deserved point when Ashley Cole's clearence hit Essien and flew past helpless keeper Hilario five minutes from time.

Ronaldo began on the bench for United but came on after the break to head his side in front after 47 minutes. The Portuguese winger then took his league tally to 10 goals when he knocked in the rebound after Chris Kirkland had saved his penalty.

Ole Gunnar Solskjaer made it 3-0 before Leighton Baines pulled one back for Wigan from the penalty spot.

"I was very pleased with the start to the second half," said United manager Alex Ferguson. "The form (Ronaldo's) in, he can always change a game."

Despite the win guaranteeing United would go into the New Year top of the league, Ferguson sounded a note of caution.

"We've got a lot of roads to travel yet and there will be points dropped, United and Chelsea will both drop points," he said.

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