LONDON (REUTERS) - Olympiakos Piraeus were too open in the Champions League defeat by Manchester United and West Ham United will be more cautious when the teams meet at Upton Park on Saturday, said manager Sam Allardyce.

United beat the Greek champions 3-2 on aggregate to reach the Champions League quarter-finals, overturning a 2-0 first-leg deficit with a Robin van Persie hat-trick on Wednesday night.

West Ham boss Allardyce was critical of the tactics employed by Olympiakos coach Michel.

"He was 2-0 up and had he sat back with a defensive blanket of two banks of four then I think United would have found that much more difficult to break down," he told a news conference on Thursday.

"Opening himself up let United exploit those spaces and there's nobody better to do that than Wayne Rooney and Robin van Persie.

"They didn't have to score a goal to get through to the quarter-finals, all they had to do was not concede one, but he never set his tactics out like that."

West Ham are 12th in the Premier League, six points clear of the relegation zone with nine games left.

Allardyce believes seven more points will see his team to safety and securing their first league win against United since 2007 would be a good start.

The West Ham manager is wary of the visitors who have struggled this season and suffered an embarrassing 3-0 home defeat by bitter rivals Liverpool in their last Premier League game.

"I think they're always very dangerous when they're not doing as well as they should," said Allardyce.

"There's a lot of people saying United are playing better football away from home than they are at home because there's not quite so much pressure on them.

"The last away game they played against a Premier League side they won 3-0 at West Brom so I think it's going to be a difficult game with the talent they've got. They can always come out and deliver at any specific time."

Seventh-placed United are likely to be without striker van Persie on Saturday after the Dutchman injured his left knee against Olympiakos.