It was a demolition from City, who were three goals to the good before half-time as Marco Silva's side were on course to suffer a damaging home defeat.
"He'll be so important this season", Guardiola told reporters in Manchester on Friday.
City are white hot at the other end, scoring nine goals in their last two games without reply.
Richarlison had the first decent chance of the game but with City guilty of some awful defending, the Watford star sent a free header wide.
Silva, who has hit the ground running with eight points from a possible 12 since joining Watford, will hope his side can replicate their recent defensive solidity. The Argentinian striker compounded Watford's regrets by scoring his second from close range in the 31st minute; he could barely miss after Gabriel Jesus blazed down the left, then released David Silva on his outside.
With a well-slicked City attack featuring Sergio Aguero and Gabriel Jesus supported by the likes of Kevin De Bruyne, David Silva and Bernardo Silva, Guardiola believes he could be on the brink of another era-defining run, with a Premier League trip to Watford on Saturday their next examination. The first game of the season was against Liverpool, at Vicarage Road, and could be the blue print for how they'll play against City.More news: Hand in Hand telethon raises $14M for Harvey, Irma victims
"We know how much quality we have", said the 26-year-old Belgium global.
We can see a lot of goals in this game and are backing on more than 2.5 goals as our prediction. "The big clubs win the games and that is the big difference to the big teams", he added. However, they are yet to win a game at home this season as their second home game ended in a goalless draw with Brighton.
The skipper's Premier League season has been limited to two brief substitute appearances and he will be desperate to get his first start and prove he can still be the main man.
Manchester City look like an easy pick in this match, but Watford have started the season off strong and could surprise City at home.
Meanwhile, Raheem Sterling is available after serving his one-match ban last week, and is in contention to make the starting lineup with Guardiola likely to shuffle his pack to keep all his charges fit and firing.
If the Hornets secure all three points, they would be at the summit of English football's top flight for the first time since 1982.