Former Aston Villa defender Paul McGrath thinks that Manchester United boss Sir Alex Ferguson needs to stop tinkering with his side's back four.

The Premier League champions have conceded five goals in their last three matches, with Rio Ferdinand and Nemanja Vidic constantly in and out of the set-up due to injuries.

"It's never been settled at the back this season," McGrath told talkSPORT. "The change has to come (replacing Ferdinand and Vidic) and Sir Alex Ferguson has always done that gradually but he's been a little pushed into throwing the lads in.

"They look a little unsettled though because it has changed so much, with Vidic and Rio being out and then coming back in.

"That's upset the back a little bit and it's shown. They've conceded a few more than usual, and they're sloppy goals as well, and that's not usual."

The Red Devils take on Aston Villa this weekend after they were knocked out of the Carling Cup by Crystal Palace in the quarter-final last night.