Foster picked the ball out of his net a whooping eight times as the champions inflicted upon the Hornets their heaviest ever Premier League defeat.
Foster was not in goal when City demolished Watford 6-0 in last season’s FA Cup final as Heurelho Gomes took his place between the sticks on that occasion, but the former Manchester United goalkeeper had a worse day at the Etihad on Saturday.
Watford were already down by five goals by the time the clock hit the 20th minute mark before they shipped three more after the break.
As Foster watched proceedings unfold before his eyes, he was left amazed by how the champions carved Watford apart and has warned the rest of the Premier League teams they could soon reach double figures of goals in a game.
"They were scary at times," the former England international told reporters.
"It was just relentless. As a goalie, coming here in the first place it's almost an achievement to keep it at one or two goals. You could see them opening us up and the chances developing in front of your eyes.
"It was absolutely no fun. In all honesty, it could have been double digits. They will do it to somebody, there will be a nine or 10 out there soon."
"Nobody gets a second to think about it," he added.
"Our players are getting the ball passed to them and before they know what to do with it they've got a Man City player attacking them. They all do it together. Their shape is incredible.
"They're all absolute specimens of men. If you look at them, they're all the same sort of build, they're not the tallest but they are all so strong and rapid. Their acceleration and touch is incredible.
"But that’s what £70million or £80million buys them."
Watford are winless this season and are bottom of the Premier League.
