A superb half-volley from Saido Berahino was enough for Tony Pulis' men to collect all three points in Saturday's Premier League encounter at The Hawthorns, as a below-par Southampton slipped to a second consecutive league defeat.
Southampton remain in fifth despite the loss, albeit they will drop to sixth if Liverpool beat Manchester City on Sunday, and Koeman admits his team are failing to match the sort of early-season performances that made them Champions League candidates.
"I was always realistic, never I was thinking that we would qualify for the Champions League because [there are] a lot of big teams who normally fight for that," he said.
"We try to win more points than we did last season and if we play our way of playing then we will see what happens.
"We are not focused on the position on the table, we are focused on our football. That's the best solution and at this time it's still the best solution.
"The total performance of team in quality is a little bit low from the beginning of the season, but we have to go and believe in the next win."
Koeman was left frustrated by what he perceived to be time-wasting tactics from West Brom, but conceded that his team did not deserve to return to the St Mary's Stadium with three points.
"We were looking for action [from the referee] over the throw-ins and free-kicks. They took too much time," he added.
"I think it was every time about 13 seconds and we spoke to the referee but they didn't do anything about that - maybe one yellow card. But that isn't the reason we lost.
"We know the opposition. They play out of a very good defensive organisation, very defensively, but they need points and they deserved the win.
"For us it was too difficult to break down. They scored a great goal and we complain more about our football. We may be had one or two chances to score a goal but not more."
