Matip was one of the central defenders who started the game, alongside record signing Virgil van Dijk. However, the defence was unable to keep West Brom out, with the away side scoring three goals and eventually seeing out the game.
Matip apologised for his part in the result.
"We started in front, our offensive play was okay, but in the defence we had problems," Matip told Liverpool's official website.
"Mistakes like my one kill every team. This mistake cannot happen, I have to clear this ball.
"In the first half, [conceding] three goals, it hurts every team. It was really hard to come back then.
"We have to do everything to turn things [around], to get in better shape and get better results."