Jimmy Shan admitted Albion gave themselves a mountain to climb at Ashton Gate on Tuesday night.
Bristol City raced into a three-goal lead inside 20 minutes courtesy of strikes from Josh Brownhill, Andi Weimann and Jack Hunt.
The Baggies delivered a spirited second-half showing and pulled two goals back through Dwight Gayle and Jay Rodriguez, however, they were unable to find an equaliser and Shan says their slow start to the encounter gave them too much to do.
He said: “The nature of the goals were very soft.
“If you’re three goals down after such a short period of the game, you’re always facing an uphill battle.
“If you concede goals so early on the opposition’s momentum grows.
“I thought we could have been a bit more aggressive.
“We cannot concede three goals that early in the game and in the nature we conceded them.
“I asked for a reaction at half time and we got one but ultimately we left ourselves with far too much work to do after our start to the game.”