Stephane Sessegnon had made it 2-1 to Albion when the ball squirmed through Cech's grasp and into the net.
And, although Eden Hazard converted a controversial late penalty to rescue a point, the experienced Czech Republic shot-stopper feels the Blues would have won if he had not lost his footing at such a key moment in the match.
"There was no indication of us not winning," Cech told his personal website. "However, shortly after the interval we conceded a cheap goal from a corner.
"Then we let them go 2-1 up because of my mistake. I lost my footing on the pitch and then I failed to react to the shot properly. It was a cheap goal to concede.
"One point is better than nothing, but that doesn't change the fact that we should have won and put the advantage of playing at home to good use."