Keane, a driving force in United's historic 1998-99 season when, uniquely, they won the Champions League, Premier League and FA Cup treble, reckoned City's humbling at Anfield highlighted how "hype" surrounds Guardiola's creation.
City can lift the Premier League title at an earlier stage of the season than any team before them by beating Jose Mourinho's United on Saturday at the Etihad Stadium.
Yet their former midfield powerhouse Keane believes the Liverpool hammering demonstrated just how far the much-lauded champions-elect still had to travel before being considered a great side.
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Keane told ITV's Champions League highlights programme: "There has been a lot of hype about this Manchester City team, talk about them being a great team.
"It was a reality check for this team. They have a lot to do yet before they want to be regarded as a great team."
Keane was a key component of a Champions League-winning squad in 1999 which many believe to be the best side to win the English title in the Premier League era but which never annexed the title in the same dominant manner as City this season.
Yet the Irishman felt Guardiola's men looked overwhelmed in the hothouse atmosphere of Anfield.
"You look at City players there, they will be shell-shocked. There is no way they would have visualised being 3-0 down after the first leg tonight," Keane said.
"Even if they thought they were in for a tough night, I'm sure they thought they were going to score a goal, so they will be shell-shocked.
"It'll be a hard game for the manager to analyse. After the game he said they had a lot of possession, but possession doesn't win you football matches."
(Reporting by Ian Chadband, editing by Pritha Sarkar)
