Mark Bosnich believes that Manchester United manager Sir Alex Ferguson should sign a defender during the January transfer window.

Failure to do so could cost the Red Devils dearly in the Premier League and Champions League, according to the former United goalkeeper.

"Teams aren't as afraid of going to Old Trafford as they have been in recent times and Benfica showed that," he told talkSPORT. "They are suffering from defensive lapses and they're going to have to eradicate them if they're going to go further.

"I wouldn't be surprised if Sir Alex goes into the market In January to strengthen the defence. It doesn't look the same defensive outfit, and in the Champions League they've come unstuck against teams who aren't big European teams.

"If they come up against a Barcelona or Real Madrid and defend like that they'll be picking the ball out of the net like they did against Man City."

Bosnich, 39, spent two seasons with United up until 2001.