LONDON, Dec 26 - Chelsea's recent defensive frailties returned to cost them two valuable points in their quest for a hat-trick of Premier League titles on Tuesday when they drew 2-2 at home to Reading.

In the absence of influential captain John Terry, who is sidelined with a back injury, Chelsea have now conceded six goals in their last three league matches.

Late winners against Everton and Wigan Athletic rescued 3-2 victories, but on Saturday their luck deserted them as Michael Essien scored an own goal five minutes from time.

Chelsea manager Jose Mourinho said Terry's continuing back problem was becoming a concern.

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

"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. In 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."

The last time Reading and Chelsea met, in October, the game was marred by a serious head injury to Chelsea keeper Petr Cech after a collision with Steve Hunt.

Hunt, who received death threats after the incident, was booed loudly when he warmed up as one of Reading substitutes at Stamford Bridge, but Chelsea's fans soon became preoccupied by their side's below-par display.

After a scrappy opening 35 minutes, Drogba broke the deadlock when he muscled through to head in Frank Lampard's corner.

WARNING SIGNS

The warning signs were there for Chelsea when Steve Sidwell smashed a shot into the side-netting as Reading started the second half strongly and it was no surprise when Leroy Lita stooped to head in Glen Little's 67th-minute cross.

Drogba restored Chelsea's lead five minutes later, powering in another header, this time from an inviting cross delivered by substitute Shaun Wright-Phillips who had replaced the ineffective Andriy Shevchenko.

Reading's Kevin Doyle missed a good chance when he shot over the crossbar but he was involved again for his side's equaliser after 85 minutes, his low cross being cleared by Ashley Cole straight into Essien and the ball flying past the helpless Hilario.

Elsewhere Portsmouth moved third with a 2-1 victory at relegation-haunted West Ham United, Linvoy Primus netting twice in the first half at Upton Park.

Tottenham Hotspur claimed a 12th successive home victory when Jermain Defoe struck twice in the 2-1 defeat of Aston Villa at White Hart Lane.