With his side leading 1-0, Foster came off his line in the 80th minute to attempt to punch clear a cross. However, he was beaten to the ball by City striker Edin Dzeko, who headed in to level up proceedings.
"The first goal was a horrible one," Foster told reporters. "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'.
"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."
West Brom went on to lose the match 2-1.