Sir Alex Ferguson believes that the fitness of his central defenders will be integral to Manchester United's fortunes in the Champions League this season.

The Red Devils have been struck by a series of injury problems at the back, with Phil Jones and Chris Smalling currently absent with knee and foot problems respectively.

Rio Ferdinand and Jonny Evans have also spent significant time on the treatment table in recent seasons, while captain Nemanja Vidic missed the second half of last campaign with ligament damage.

Ferguson highlighted these injuries as a telling factor in their 2011-12 group-stage exit after United conceded a total of eight goals in costly defeats to Benfica and FC Basel.

The Scot has claimed that keeping his defenders fit this time around will be key to the success of their European campaign, which begins against Turkish champions Galatasaray on Wednesday.

He said: "I think the key is to keep the defenders fit. I mean that. We had a rough ride with defenders being injured in the last two or three years.

"If I could be guaranteed two would be fit for the rest of the season, I'd gladly take that right now. If they remain fit that gives us a better opportunity.

"But that's not been the case over the last few years. It's a key issue for us and we can't hide it. I just hope we get a bit of luck in that respect."

United kept their first clean sheet of the Premier League season in their 4-0 victory over Wigan Atheltic on Saturday.