Manchester United manager Alex Ferguson could never have imagined the loss of Rio Ferdinand would have such an impact on his team's Premier league title challenge.

When the England defender started his last match at Wolverhampton Wanderers three weeks ago before beginning an eight-month ban for missing a drugs test, United were a point ahead of Arsenal at the top of the table.

Since then, the champions have conceded eight goals in three matches, still managing to beat Southampton 3-2 and Everton 4-3 before Wednesday's 3-2 home defeat by Middlesbrough left them five points behind Arsenal.

"We can't afford to lose another game," Ferguson said. "We've been slack in defence and if we keep doing that, we can say goodbye to the league."

Ferguson described Middlesbrough's goals as "soft" after diminutive Brazilian Juninho twice exploited poor marking to score with headers and Joseph Desire Job evaded two defenders to clinch victory.

But Ferguson should not be too surprised by his side's defensive frailty. Since the days of Steve Bruce and Gary Pallister in the early-to-mid 1990s United have never consistently fielded a reliable central defensive pairing.

Dutchman Jaap Stam, Norwegians Ronny Johnsen and Henning Berg and France's Laurent Blanc all filled the void with varying success but it was not until Ferdinand and Mikael Silvestre were paired this season that a genuine partnership was formed.

Ferdinand was beginning to find his feet following his £30 million transfer 18 months ago, while Silvestre had wanted to move into the middle from left back, the position he occupied for most of his first four seasons at the club.

United conceded only 14 goals in 21 matches and when it became clear Ferdinand faced a lengthy suspension Ferguson brushed off concerns by saying Wes Brown would step in after recovering from injury. Brown duly returned to the side after six months on the sidelines but has looked rusty and Ferguson lost patience when he substituted him at half-time against Middlesbrough.

The real problem for Ferguson, however, is that he has few options. Full backs John O'Shea and Gary Neville can play in the centre and midfielder Roy Keane could also drop back but all three offer more in their regular positions and it remains a mystery why Ferguson chose not to obtain reinforcements during the transfer window.

Middlesbrough's Gareth Southgate would have been a perfect choice and the England international gave another display of canny defending in his side's victory at Old Trafford. Ferguson is aware of the problems but has missed the opportunity to do something about them. "It is not a quirk of fate to be conceding so many goals from situations like that," he said. "We are giving the opposition too much room and that is very disappointing."

Although United's attack is still sharp with the goal threat of Ruud van Nistelrooy, Louis Saha, Ryan Giggs and Paul Scholes, they cannot hope to win a ninth title in 12 seasons, let alone the Champions League, with such a porous back line. Ferguson, distracted by off-pitch controversies, will need all the experience gained in more than 17 years at United to solve that particular problem.