Wes Brown has conceded Manchester United will have to defend far better at Stamford Bridge on Sunday if they are to get anything out of their blockbuster confrontation with Chelsea.

Both sides will go into the game with their Champions League knock-out place secure following draws on Tuesday night.

But while Carlo Ancelotti's men were probably pleased with their efforts against Atletico Madrid, United looked far less secure.

It took another injury-time strike, this time a deflected Antonio Valencia effort, to earn a 3-3 draw against a spirited CSKA Moscow outfit at Old Trafford.

The ease with which the Russians scored their three goals provides huge cause for concern, especially as Sir Alex Ferguson has claimed Rio Ferdinand is highly unlikely to return as he struggles to get over a calf injury.

It means either Brown or Jonny Evans will continue in central defence alongside Nemanja Vidic, who should be fit.

And Brown knows against the likes of Didier Drogba and Nicolas Anelka, such defensive weakness will be punished.

"We cannot concede like that when we go to Stamford Bridge," he said.

"We have to be tighter at the back and give the strikers the chance to do a bit of damage at the other end."

It is another key game, coming just a fortnight after the defeat at Liverpool which cast fresh doubt over United's ability to sustain a title challenge this term.

If anything, Brown expects Sunday's encounter to be even tougher.

"It will be our toughest game of the season," he said.

"They have been doing very well.

"They are a well organised team and they will be very strong. They are scoring goals at the moment and we will have to be really on top form to think of getting any points."