Newcastle United boss Eddie Howe was left to rue a defensive collapse as his side were dumped out of the Champions League round of 16 by Barcelona in spectacular fashion. After competing valiantly for much of the first leg and parts of the return fixture, the Magpies conceded seven goals on the night to fall to an 8-3 aggregate defeat. Howe pointed to a lack of concentration as the primary reason for the capitulation in Catalonia, as Barcelona cruised into the quarter-finals. After a 1-1 draw at St. James' Park, the second leg saw Raphinha involve himself in six goals, while Robert Lewandowski, Marc Bernal, Lamine Yamal and Fermin Lopez all found the net.“The defending today was not on the level that it was just a few days ago at Chelsea. Starting with the first goal, where two players slipped, then we concede a set play, then probably the big one was the penalty," Howe admitted.Barcelona took an early lead through Raphinha, but Elanga twice brought Newcastle level, including a clinical finish following a mistake by Yamal. Despite Elanga’s brace, the Blaugrana consistently found answers through Bernal and a stoppage-time penalty from Yamal - earned after Kieran Trippier fouled the ever-present Raphinha - to ensure the hosts stayed ahead in a frantic first half.“So as well as we played in the first half, I thought we were outstanding in many aspects. It was really a great representation of how we want to play. There are too many individual errors within the performance to carry that great performance through," Howe noted. Despite the early promise, the second half saw the floodgates open as Barcelona found another gear.Howe was perplexed by his side's defensive collapse as Hansi Flick’s men scored three times in just 10 minutes after the break. Clinical finishes from Fermin and a Lewandowski brace exposed every flaw in Newcastle's backline, before Raphinha added a seventh following a Jacob Ramsey error. This ruthless display completely undermined the Magpies' tactical discipline, leaving the manager questioning their uncharacteristic lack of concentration.Howe explained: “It was really, if we performed anywhere near, or defended anywhere near than we can, I think we'd be leading at half-time. I didn't see a need to necessarily change how we were playing at half-time. I thought we were really, really strong. But then of course we concede another set play. The first four goals we're looking at again, in terms of how we play, it's a strange execution of the goals against us.”Add GOAL.com as a preferred source on Google to see more of our reportingBy the final whistle, the scoreline reflected a gulf in class. Howe was honest about his team’s inability to maintain their physical output or recover from the psychological blow.“I don't think psychologically you can say that we recovered from the moment at half-time. That's what it looked like to me. In the second half we certainly didn't have the same energy. And when the game was effectively over, we hadn't lost that level either. So we came in very, very well in the second half. And even changing to play lower and deeper and compact space didn't really help us at all," Howe concluded.