"Our energy level was simply not good enough today, especially in the first half," Bilic admitted. "Second half was good, we can talk about the quality of our crosses and our one-on-ones, we can talk about the decisions in the final third.

"We can't talk about the determination and energy level in the second half because we had it. In the first half, we were slow, nowhere near them.

"They had time and space to create, so we had that problem. We weren't 'crazy' for it, and when we had the ball we lacked the quality. We have to admit today we lacked a bit of everything, and we got punished.

"It's not nice, it's not what we wanted. This group have made it together and it's only us who got it back.

"All I can blame on that first-half performance is complacency.

"It's not deliberate, we weren't prepared for any other games. OK today's a bit easier and Wednesday will be a bit harder – no no no no.

"It's a great thing to have a game in a few days, to make up for what was lacking today."

On the other hand, this was a particularly significant result for Boro, and for manager Jonathan Woodgate. This was their third league win in a row, and their first away from the Riverside.

Woodgate, who was without a number of first-choice players at The Hawthorns, fielded a young side but got the display he asked of them, as they denied their hosts and took advantage at the other end.

Still, the former defender called for level heads.

"Let's not get carried away. We're keeping our feet on the ground. We've got Preston on New Year's Day so that'll be a real test," he said.

"It's a top result against a real top team who will probably go on to win promotion.

"We've changed shape regularly this season, but I felt it was important we dealt with (Matheus) Pereira. He's their best player by a mile, so I thought he had to nullify him by putting the two holding midfielders around him.

"We only had one day to work on this shape, really, but we showed them videos of exactly what we wanted. On the break we looked really really dangerous, and we nullified their threat.

"I don't think too many teams will come here this season and do that. We pressed them high up the pitch, we played two defensive midfielders to nullify Pereira.

"He's their best player, he's top drawer. The players gave me everything out there. We've drawn five games away from home, but we could've easily won three of them.

"We weren't coming into this game really struggling, we just needed that win. We knew it would come sooner or later."