Slaven Bilić admitted his Albion side “simply weren’t good enough” as Middlesbrough left The Hawthorns with all three points on Sunday.
The Baggies, who suffered their first home defeat in the league since February, were beaten by a goal in each half from Daniel Ayala and Ashley Fletcher.
And the boss stressed the importance of returning to the performance levels which saw Albion rise to the top of the Sky Bet Championship table.
“The first half was very flat," said Slaven.
"We were not aggressive enough, we did not have the circulation of the ball that we normally have. Up front we were not dangerous enough when we had the one on one occasions - and they deserved it.
“Second half we improved a lot. We can say that still we lacked that quality with the crosses. The crosses were quite poor today. A couple of them were good and we had a chance from Charlie and their goalie made a brilliant save. Overall it was not the performance. The result is what it is.
“We have to pick ourselves up. I told the players of course we are not happy and in the first half we were nowhere near them really. They had enough time and space to pass the ball and they had a few chances, not only the goal. We got what we deserved to be fair.
“It is a shame that we end the year with a defeat at home, the first one after a long, long time. We have to pick ourselves up because we have a big game up next.
“We are not talking about 25 games, we are talking about this game and we have to be realistic and say we were not good enough. Simple as that. We have to find a way very quick to come back and be what we used to be very recently.”