City striker Mario Balotelli, who scored both of City's goals in the 2-0 win, was handed a second yellow in the 63rd minute for a tangle with West Brom's Youssuf Mulumbu who was shown his first caution for the same incident.
Di Matteo said referee Lee Probert got the decision correct to send Balotelli off and then give Mulu mbu his marching orders for his 82nd minute challenge on Carlos Tevez.
"I think the referee was in a good position on the (Balotelli) sending-off," Di Matteo said.
"My player wasn't happy with the challenge and the official was there.
"In terms of the second one, for my player, I think there was no doubt about it."