The Blues' manager has criticised his defenders following the shock defeat against the Baggies, imploring his back-line to be more "defensively attentive"
Goals from Shane Long and Peter Odemwingie subjected the Blues to a fourth league game without a win and the loss leaves them four points adrift of league leaders Manchester City.
Di Matteo laid the blame at the door of his defenders, having seen his side fail to keep a Premier League clean sheet since September.
"Defensively we were not good enough to win the game after conceding two goals. Thinking that you need to score three goals to win the game is just mad," Di Matteo told reporters.
"We are trying to find the right balance where we can create chances and win games, but we have to start with being more defensively attentive."
The Italian defended his decision to rest key players against his old side as Ashley Cole, Oscar and Juan Mata, among others, were all on the substitutes bench.
"It was forced on us with the international games in midweek. We had players coming back from long trips overseas with five or six hours of time difference," Di Matteo continued.
"They were very fatigued and tired and were not in condition to start the game."