Irvine made no fewer than 10 changes to his first XI from Saturday's 2-2 draw with Crystal Palace as the Baggies went down to an 86th-minute winner from Callum Wilson.
"It was a team full of first-team players," he told reporters. "A team who train as a first team every day and it was an opportunity for them.
"They need to grab those opportunities and they need to get the benefit of the game as well. We have a squad of players who are all Premier League players, they are all players who play at a higher level and are internationals.
"It wasn't a team of kids, it was a team that if they played to their best of ability should have been able to get a result."
Eunan O'Kane had fired the Cherries into the lead on 49 minutes, before a Tommy Elphick own goal five minutes from the end looked to have saved West Brom.