The former England international was at fault for the Reds' first goal as the away side came from behind to beat the Baggies 2-1 at The Hawthorns.
While Clarke admits that the Merseysiders were the better team, he feels that Foster's mistake had a big part to play.
"We said we'd have a go, we certainly started that way. I think their first goal knocked the stuffing out of us a bit," said Clarke. "I don't know why, maybe because it was such a soft goal to lose, it knocked us back a bit in the first half.
"It is unusual for Ben. I don't think it's even worth having an in depth discussion about it. It is not like Ben, it doesn't happen very often and we move on.
"After 1-1 they were better, they passed it better, we sat off them too much. The second half was much better, we got about them, we pressed, made a game of it, and the second half could have gone either way."
Albion face Aston Villa on Sunday.