The Baggies held a one-goal lead and a one-player advantage up until the 80th minute against the reigning champions but crumbled as Edin Dzeko came off the bench to score twice to force a 2-1 result to City.
Foster has shouldered the blame for the defeat by accepting he was at fault for Dzeko's opening header before the Bosnian struck with a neat first-time effort but he also commented on the cost of City's players and how that equates to class in front of goal.
"It was a great ball in, to be fair. I thought I'd come and get something on it but once you are on your way you think 'oh no, here we go,'" he told West Brom's official website.
"You are hoping the defender gets it but it was a great ball in and a great header as well. I should have stayed on my line.
"The second one is credit to us that we were trying to win the game against Manchester City.
"We were giving it a go at least. They broke on us and that is what £70 million worth of talent does for you. I'm gutted. I thought we played really well.
"But we will just carry on the way we are going. We are playing fantastic football and we are going to win a lot more games than we lose if we carry on playing like that."