The Championship side sealed a 1-0 win at the Amex thanks to Adam Reach’s second-half goal and despite some late pressure Albion couldn’t find an equaliser.
Duffy was one of seven alterations to the team that drew against Chelsea and admitted it was a missed opportunity for last year’s semi-finalists.
“We didn’t really get going,” he admitted. “They set up to frustrate us and we were a bit sloppy at times - on the day we just weren’t good enough. The manager made some changes and it was a chance to impress so it’s disappointing.
“We had a couple of half-chances in the second half but they defended well and were well organised.
“It is going to hurt for a couple of days but we have to regroup next week and concentrate on the Premier League.”
For Albion that means a trip to Duffy’s old club Everton next Saturday and the defender is confident of a positive reaction from Albion.
He added: “A lot of players wanted to put their name in the hat for Everton but we have to pick ourselves up and produce a reaction against Everton. We normally get that after a defeat and I’m sure we’ll get one on Saturday.”