Sir Alex Ferguson
Sir Alex Ferguson backs Louis van Gaal to do well at Manchester UnitedGetty

Former Manchester United manager Sir Alex Ferguson says the Red Devils' back four looks fragile this season.

The 20-time English champions allowed Nemanja Vidic and Rio Ferdinand to leave the club on a free transfer after the end of the last season.

In addition to this, left-back Patrice Evra completed his switch to Juventus earlier in the summer, despite signing a one-year extension in May.

United added Luke Shaw and Marcos Rojo to their squad in the summer transfer window. However, injuries to first choice centre-backs Phil Jones, Jonny Evans and Chris Smalling saw Louis van Gaal field youngsters Tyler Blackett and Paddy McNair in the back four.

Ferguson has asked United defenders to be more consistent in order to avoid dropping further points this campaign.

"It's been a bit of one step forward and two back. It's been difficult with injuries. Phil Jones is injured again," Ferguson explained.

"I said, 'Are we going to get a break here?'. If the back four doesn't play consistently together then you aren't going to get results."

"People keep saying 'you're fragile at the back', and that's understandable. But these boys need time to adapt, Luke Shaw and Marcos Rojo are still young men," he said.

After Ferguson stepped down from his managerial duties in May 2013, United appointed David Moyes as his successor. However, the Red Devils struggled under the former Everton manager and they finished seventh in the league table last season.

United sacked Moyes earlier in April this year and Van Gaal took charge at Old Trafford after completing his term with the Netherlands till the World Cup.

The Old Trafford club are eighth in the table with 13 points from nine matches and are three points behind fourth place West Ham United. Ferguson believes Van Gaal's stubbornness will help him succeed at United.

"He (Van Gaal) will get there because he's got the stubbornness and determination to do well. I have no doubt he will get it right,' he said. 'I am certain they made the right choice," Ferguson stressed.