Man City aren't one of the Premier League's best ever - Rooney
Pep Guardiola's side thumped Tottenham 4-1 on Saturday to record a 16th straight win and open up a 14-point lead over Rooney's former club Manchester United before their clash with West Brom on Sunday.
Rooney scored in Everton's 1-1 draw with City in August – the only Premier League game that they have dropped points in – and the former England captain is convinced that Guardiola's side will drop further points over the remainder of the campaign.
"I'm not sure they are quite there. They would have to keep going and do a lot more," he told talkSPORT.
"There have been so many great teams throughout the years in the Premier League. This City team needs to win trophies and needs to do it for a longer period to be in that bracket.
"The best ever? I would go with the Man United team from 2008!
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— Wayne Rooney (@WayneRooney) December 10, 2017
"I don’t think they can go unbeaten. They have done incredibly well with the run they have been on, but the Premier League is so difficult, especially now with this Christmas period coming up.
"I know they have a huge squad, a great squad of players. But there are so many top teams. I think in previous years it has been two teams and you went in knowing you would win most of the other games.
"But now there are five or six other teams that can all beat each other and I'm sure in one of those games Man City will get beaten."
Kevin De Bruyne, Leroy Sane, Raheem Sterling and David Silva have been winning plaudits for their sparkling displays this season, but Rooney has heralded the understated brilliance of midfielder Fernandinho.
He added: "Fernandinho gives the other players license to go and play, go and express themselves. Those players go under the radar a little bit.
"Without him, the likes of Silva and De Bruyne start worrying if they lose the ball. Fernandinho gives them great cover."