Manchester United goalkeeper David de Gea has said the team's poor form is due to a "lack of quality" in certain areas.

United have won only two of their last eight Premier League games and sit ninth in the table, nine points behind the Champions League places ahead of Sunday's match at home to Aston Villa.

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"I don't think it's through any lack of effort, the boys are leaving everything out on the field in training and games," De Gea told Sky Sports. "You could point maybe to a lack of quality in certain areas, but certainly not a lack of fight."

United's 3-3 draw at Sheffield United last weekend left them with 17 points from 13 games -- their worst start to a Premier League season -- and De Gea said they needed to win a few games on the trot to find some measure of consistency.

"We need to win four or five games in a row to be at the top, but at the moment the team is not playing so well," De Gea added.

"We're giving it everything, we're trying to get those big wins that can put us high up in the table. But the truth is at the moment we're not consistent enough, there's much to train and much to improve."

Ole Gunnar Solskjaer's side host Tottenham Hotspur -- managed by his predecessor Jose Mourinho -- on Wednesday followed by a visit to champions Manchester City next weekend.