Louis van Gaal admits reviving Manchester United's fortunes could be the hardest job of his 23-year management career.

Van Gaal has won league titles in Spain, Holland and Germany, well as all the major cup competitions in Europe, but he is finding success hard to come by in his latest post.

After five matches, Van Gaal's United have just five points - their lowest tally at the start of a campaign since the Premier League's inception in 1992.

United conceded four goals in 30 minutes as they crumbled to a 5-3 loss to Leicester last weekend, and Van Gaal's hopes of an immediate upturn in form have been hit by an injury crisis to his creaky defence.

The Dutchman, who has just one centre-half fit for Saturday's game against West Ham, believes bringing the glory days back to Old Trafford may be the hardest task of his glittering career.

"At this moment it is may be yes [my toughest job]," Van Gaal told a press conference.

"As a manager you know in advance it's a big challenge and the club is in a transition but you don't know in advance the difficult moments."

Some of the attacking play from United's Galacticos was simply stunning at Leicester last week, but the team's defensive deficiencies cost them dearly.

United gave away two penalties and slack marking allowed Premier League newcomers Leicester to bang in three more goals at the King Power stadium.

Former United captain Gary Neville said Van Gaal's team looked "soft-centred", while Ruud Gullit questioned why his fellow Dutchman had spent so much on the likes of Angel di Maria and Radamel Falcao when the team was so vulnerable at the back.

Van Gaal insists United's defending has not been as bad as everyone makes out though.

He said: "When you look at our figures in the Premier League, apart from the goals at Leicester we are defending well.

"The average is less than one per game. We are scoring a lot of goals, too. We are fifth or sixth in terms of scoring."

When asked whether he was still confident of achieving the goal of a top-three finish set by the board, Van Gaal said: "Yes.

"We should have won last weekend.

"If we had we would have been two points off second place in the table. Because of the loss you are now questioning me.

"If we'd have won it would be different. You can't make one incident a regular one. We will overcome things. The season lasts more than one competition day."

Any hopes for an immediate response on Saturday have been hit by a spate of injuries at the back.

Van Gaal confirmed Jonny Evans suffered severe bruising of his right ankle following a tackle with Leonardo Ulloa at Leicester last weekend and will join the suspended Tyler Blackett in missing the visit of Sam Allardyce's West Ham.

Phil Jones is out with a hamstring injury and Chris Smalling is also unavailable after damaging his thigh in training on Wednesday.

Given that Marcos Rojo is now his only fit senior centre-back, Van Gaal is considering handing a debut to 19-year-old Academy defender Paddy McNair.

Van Gaal said: "All the three right-sided centre-halves are (unavailable).

"I don't know how long they will be out for.

"Evans is a very difficult. His bone is bruised a lot and we have to evaluate it.

"Smalling ran out of the training session because he had stiff legs.

"We have to see what that means. But I don't take risks with players."

Van Gaal denied he would be taking a gamble if he deployed one of his youngsters against a West Ham team buoyed by their 3-1 win over Liverpool last weekend.

"The younger players can have their chances now. That is also the policy of this club," he said.

"They are always ready.

"They have hunger to play the game and Manchester United youth education has a certain level that can provide that.

"It's a big difference (between youth football and the Barclays Premier League), but in one game you can show yourself."