Sir Alex Ferguson has admitted that his defensive options remain limited ahead of Manchester United's Champions League clash with Besiktas on Wednesday night.

United have already qualified for the last 16 with two group games still remaining so Ferguson has the luxury of resting some key players against the Turkish champions.

However, he looks set to name the same back four that started Saturday's Premier League game against Everton as several defenders remain sidelined by injury.

Nemanja Vidic, Rafael, Wes Brown and Patrice Evra were the only four defenders to take part in training on Tuesday.

Rio Ferdinand, John O'Shea, Jonny Evans and Fabio have all been ruled out, while skipper Gary Neville is doubtful.

"They're all out," revealed Ferguson. "Really it's a fight to get one of them for Saturday's game (against Portsmouth). But I have my doubts.

"Sometimes it happens and you get good runs free of injury and other times you don't.

"That's the value of having a good squad. It'll probably be the same back four as Saturday against Everton.

"Gary Neville didn't train today but I'm hoping he'll train on Wednesday so we will see how he is after that.

"We have plenty of options in midfield and in the front positions. It's just the back four that is causing us concern. I hope I get one or two more fit for Saturday."

Darron Gibson, Gabriel Obertan and Ji-sung Park are all expected to start for the Red Devils at Old Trafford.