The Hornets felt the full force of the champions' attacking power on Saturday as they were dismantled at the Etihad Stadium, just four months after they were overpowered 6-0 in the FA Cup final.
Pep Guardiola's men have also registered a 9-0 and two 7-0s in knockout action in 2019.
"They will do it to somebody," Foster said. "There will be a nine or 10 out there soon. As a goalie, coming here in the first place it's almost an achievement to keep it at one or two goals. At times, if they're on it, they are incredible.
"They were scary at times. It was just relentless. 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".