Ange Postecoglou suffered his heaviest defeat since taking charge of Tottenham last summer and warned that it was unlikely to be the team’s last setback.

“I was concerned by every aspect of our game, a lot of it was self-inflicted,” said the Australian as Spurs lost 4-0 at Newcastle where the in-form Sweden striker Alexander Isak scored twice.

While Newcastle now have renewed hope of securing Europa League football next season, Postecoglou could face a struggle to qualify for the Champions League. “We weren’t brave on the ball,” he said. “It won’t be our last heavy defeat. It’s part of our growth, sometimes growth is painful.”

Related: Newcastle 4-0 Tottenham: Premier League – as it happened

With Arsenal, Chelsea, Liverpool and Manchester City among their opponents in the final weeks of the season, Tottenham will need to improve considerably on a performance that reprised memories of the team’s 6-1 defeat at St James’ Park a year ago.

“Newcastle were really good,” said Postecoglou, whose side enjoyed 73% of the possession. “We just didn’t get to any levels that would have allowed us to get a grip on the game like we usually do. It became a game of transition and we paid the price.

“Even outside the goals I don’t think our football was anywhere near the level it should be. I’m not really sure what went wrong. We lacked a lot of conviction in a lot of things.

“Newcastle hurt us, particularly in transition. We just weren’t taking control, we were allowing Newcastle to dictate. We were never near good enough. It’s tough to take and a bit raw at the moment but you can’t sit there feeling sorry for yourself, you have to look forward.”

The match featured a calamitous performance from the visiting defender Micky van de Ven. “That’s the life of an elite footballer,” said Postecoglou, shrugging. “You’re going to make mistakes. I’m sure he’s going to make more of them in his career but it’s how you react to those mistakes. I think all of us, as a group, have a fair bit to learn from today.”

Howe reaped rich dividends from his decision to switch to a back three and adopt counterattacking tactics. “It was a great performance from start to finish. We were very good in almost every area,” said Newcastle’s manager.

“To beat Tottenham we had to be flexible. They’re unique in everything they do and we felt we needed to change our game to give ourselves the best chance of winning. I thought it worked tactically. Our concentration levels we’re really good.

“Possession doesn’t guarantee you anything but I would have liked to have had a bit more than 27%.”

Howe reserved special praise for Isak, saying: “He plays for the team. He doesn’t play for himself, which is rare.” He also had warm words for Fabian Schär, Antony Gordon and Elliot Anderson.

“Fabian has been so consistent. He’s incredible on the ball at centre-half. We’ve really needed him and he deserves a lot of credit,” he said. “Hopefully we’re returning to somewhere near our best. We’ve still got a long way to go but hopefully we’re moving in the right direction again.”