London: Pep Guardiola is the world's top manager but needs more time to make Manchester City great and will have to hope for some luck to beat his former club Barcelona in the Champions League on Tuesday, says the Spanish club's ex-forward David Villa.

"Guardiola is the best manager in the world but I think it is too early to compare his Barcelona side with Manchester City. We have to give him time because I am sure he will make City a great team," Villa said.

City won their opening six matches in the Premier League this season before slumping to a six-game winless streak in all competitions - the longest of Guardiola's managerial career.

They have taken one point from their last two Champions League group games at Celtic and Barca, and need a win against the Spaniards in the return to boost their European hopes.

Barcelona top group C with nine points ahead of City on four, Borussia Moenchengladbach who have three and Celtic with one.

After City's 4-0 humiliation at Nou Camp earlier this month, Guardiola will be hoping they can keep five-time Ballon d'Or winner Lionel Messi quiet at The Etihad Stadium. However, 2011 Champions League winner Villa, now plying his trade in Major League Soccer with New York City FC at the age of 34, said Guardiola's tactics alone may not be enough to see his team progress to the next round.

"The Champions League is not an easy competition," Villa, an integral part of Guardiola's Barca side, said from New York."You're playing the best teams in the world so it's tough.

"The key to being successful in the competition is a mixture between finding consistency, playing in the right way at the knockout stages, having the best players, the best coach, the best tactics and a bit of good luck, too. It is very difficult."