The Cherries made a flying start to the match as Saints struggled to cope with Eddie Howe’s side, before Steve Cook opened the scoring on the 30-minute mark.
Ronald Koeman made a tactical change shortly after the goal, but Ward-Prowse admitted Saints’ slow start had put them on the back foot.
“We didn’t start the game well. Tactically we weren’t right and we had to have a little change midway through the first half,” Ward-Prowse told youtube.com/southamptonfc
“They started well and we didn’t. They punished us for that.
“They had a lot of the ball and space – especially out wide, which is why we had to make the tactical change.
“But we’re disappointed in ourselves because it was a game we felt we could win and unfortunately we haven’t.
“The start to the game is always important. They started better than us and had a bit more momentum,” Ward-Prowse continued.
“They got a bit of a lucky goal and that gave them the confidence.”
And Ward-Prowse insists Saints must now look to respond immediately.
“Things just didn’t go right for us here. We’re disappointed and we’ve just got to try and put it right on Saturday,” the midfielder added.
“It’s a game we felt we could win and we’re frustrated because we wanted to have bagging rights, but it wasn’t to be.
“We’ve got a few days now until the next game and we’ve got to try and put it right.”