Juventus cannot afford to switch off against resurgent Spanish side Valencia in Tuesday's Champions League game in Turin as they did against Manchester United, coach Massimiliano Allegri cautioned on Monday.

The Italian champions meet Valencia in a crunch Group H clash in Turin on Tuesday where a draw would be enough to seal the hosts' passage to the last 16.

The Spaniards lost 2-0 in Valencia in their home leg despite Juventus being reduced to ten men after half an hour when Cristiano Ronaldo was sent off.

But Allegri insists Valencia are a different side now and represent a threat to his team's ambitions of winning everything this season including a first Champions League title since 1995.

"In the Champions League you have to go step step by step. We need to break through tomorrow and try to get the top place in the group," said Allegri.

"We're playing against a team that has lost just once in their last ten matches.

"We have to be very alert and not to fall asleep like happened in the last ten minutes against Manchester United," he continued after two goals in the last five minutes gave United a 2-1 win in Turin.

"These things happen but we don't want to repeat that mistake.

"I'm not worried but I want the team to have respect for Valencia, the only thing certain is that we have to win this game."

Juventus are top of Group H with nine points after four games, with Manchester United two points behind and the Spaniards on five.

Swiss club Young Boys are already out of the running.

Valancia counterpart Marcelino García described the game as being "all or nothing" for his team against "a side that may even win the competition".

"Our future in the Champions League depends on this win. It's going to be a difficult, challenging and complicated game," said the Spaniard.