Guardiola re-signed Barca academy graduate Pique from Manchester United in 2008 and the pair won 14 trophies together in four unforgettable seasons before coach quit in 2012.
"I think Pep had an impact on everyone who played under him in those four years. I think there was a before and after with Guardiola, we understand football differently now and he changed the club's history," Pique told a news conference on Tuesday.
Barcelona have won all four competitive games against City, knocking them out of the Champions League in the round of 16 in 2014 and 2015, and the two sides meet against on Wednesday in Group C. But Pique believes the Premier League leaders are a different proposition under Guardiola.
"Pep is in charge now and he's changed the way they see and play football, which makes it a challenge for us," he said.
"They play similarly to us, we will try to use our strengths and dominate the ball, but we know City will press and play similarly to us. It will be difficult to get the ball and it it's a good challenge to demonstrate we're still on top."
The 29-year-old also reiterated his decision to quit international football after the 2018 World Cup and revealed he had considered retiring after Spain's failure at the 2014 World Cup and a trophy-less season with Barcelona.
"I spoke to people and said I didn't want to continue, but coaches convinced me to change my mind. I didn't feel motivated, perhaps because I had won so much so young," added Pique, who has won the World Cup, the European Championship, three Champions League and six La Liga titles.
"But with (coach) Luis Enrique giving us more and because I felt comfortable in the club and the national team, I realised I had to come back."
Luis Enrique, a former teammate of Guardiola with Barca and Spain, echoed Pique's comments about the current City manager.
"I can't say if he's the best coach ever but I'm sure he's going to be one of the best, someone who defined an era."
(Reporting by Richard Martin; Editing by Brian Homewood)
