"He'll be so important this season", Guardiola told reporters in Manchester on Friday.
He reserved special praise for Aguero, who scored a hat trick and again showed his class in front of goal despite questions over his future in the team and at the club.
City, meanwhile, have slammed in nine goals in their past two games and head to Watford full of confidence.
There was the memorable FA Cup game in January 2014 when Fernando Forestieri and Troy Deeney put Watford 2-0 up at the break and it was not until the 78th minute that Sergio Aguero pulled City level, and the 86th minute that Aleksandar Kolarov's put them ahead.
"We want to keep everybody fit, but so far we have not been able to play the same two centre-backs from one game to the next".
There are only three teams who are still unbeaten going into the fifth round of Premier League matches this weekend, and two of them are set to face each other today.
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The Citizens ran riot at Vicarage Road as Sergio Aguero netted a hat-trick, Gabriel Jesus and Nicolas Otamendi bagged a goal a piece and Raheem Sterling rounded off the rout from the penalty spot. On Saturday afternoon, they will host another undefeated side - Manchester City, at Vicarage Road.
Defence is the worst hit area with five key first-team players unavailable.
He could have had one more had Sterling not taken a last-minute penalty away from him after the England worldwide was brought down by Watford defender Christian Kabasele. I am learning a lot from them and I hope I can learn much more.
Watford, without a win against City since March 1989, came very close in early 2016, only for a late double to seal yet another defeat for the Hornets. Through four fixtures, they have three wins and a draw. He deserved his standing ovation when replaced by Ilkay Gundogan, whose first appearance since December previous year brought even more cheer to Guardiola. Their record is better still in the past three matches in all competitions, scoring 15 goals and allowing zero. His men will have to be at the top of their game throughout the match in order to get all three points.
Philippe Coutinho made his first start of the season for Liverpool after failing to force a summer move to Barcelona, but his return wasn't enough to fill the void left by the suspended Sadio Mane.