Beckford limped out of last night's clash at Sixfields 10 minutes after scoring his 19th goal of the season, and he was missing from the pitch as Nicholas Bignall's late header earned Northampton a 2-1 victory and plunged United below League One's play-off zone.

The forward is a major doubt for Sunday's tricky FA Cup second-round tie at Blue Square Premier leaders Histon and boss Gary McAllister was waiting to discover the severity of the damage sustained by his leading scorer today.

The loss of Beckford for a lengthy period in the lead up to Christmas would be a huge blow to McAllister, who saw the 24-year-old move to the verge of the 20-goal mark at Sixfields, and the injury completed an evening of extreme disappointment for United's manager.

"Jermaine's got a hamstring problem," McAllister confirmed. "We'll need to wait to see the degree of the problem there."

Beckford's header on 69 minutes pulled Leeds back into a game which Northampton had led following Liam Davis' 20-yard finish in the first half, but Bignall's goal three minutes from time confirmed United's sixth league defeat of the season and sent the club spiraling down to seventh in the table.

Leeds had routed Northampton 5-2 during an FA Cup first-round replay at Sixfields last week, but McAllister bemoaned a toothless first-half performance that forced his team to chase the game and the weak defending which gave Bignall a free header from a corner.

"The result's very disappointing," he said. "They got in our faces in the first half and we didn't get going at all.

"In the second half we responded well and got on the front foot, but we've conceded a bad goal again.

"Their first goal was a good one but their second was down to us not picking up players in a crucial area.

"At half-time I was looking for a response and I did get that. It was like night and day. In the first half we were put under a lot of pressure and I felt we were a bit naïve in playing a few too many passes.

"I don't think I've got players who are hiding or cheating out there and in the second half there was a massive improvement, but we have to be ready for these battles when we go away.

"I don't watch Northampton week in week out, but is the effort used by them the same every week?

"If it was then they'd maybe they'd be up in a similar position to us in the league."

The late goal conceded by Leeds was a contrast to the fortunes of other clubs around them in their division, and Oldham Athletic's last-gasp victory over Walsall pushed United out of the top six.

Leeds were closing in on the top of the table before their recent defeat to Huddersfield Town but the club now lie six points behind Leicester, who have lost only two league matches all season.

McAllister said: "I'm not at all happy with the league position. I can't be happy.

"I don't want us to be where we are in the league and the minimum we're looking for is an average of two points per game. We're behind that and we've got to get a group of victories to turn that around.

"Sometimes it's difficult to put your finger on the problem. The team who played (at Sixfields) last week were excellent and scored five goals, and this time it hasn't gone our way. It's hard to take.

"I've got the players I need but we want consistency. The second half was more like us but in the first half we weren't there."

Northampton boss Stuart Gray offered encouragement to McAllister, saying: "Make no mistake, Leeds are a fantastic team who will get promoted."

Midfielder Fabian Delph received his fifth booking of the campaign last night and will serve a one-match ban during Leeds' trip to Tranmere Rovers on December 6.