Eddie Howe says he is “confident” Alexander Isak will still be a Newcastle player at the end of the transfer window but says he sent the striker home ahead of the friendly loss to Celtic because of the focus on the Swede’s future.

Newcastle made it clear Isak was not for sale this week when Liverpool stepped up their interest in the striker. The Magpies value Isak, who scored 23 Premier League goals last season, at £150m at least.

Arsenal have been another club eyeing Isak this summer but they are now closing in on a deal for Sporting’s Viktor Gyokeres.

“It was my decision,” said Howe about sending Isak home. The striker wasn’t set to play in the game – which turned out to be a 4-0 thrashing by Celtic – because Newcastle are managing his minutes at this early stage of pre-season.

“He travelled to Glasgow with us but I decided to send him home due to the speculation around him.

“The last thing he wanted was to be sat in the stand watching – that wasn’t fair to him. But I’m confident he will be a Newcastle player come the end of the window.”

Howe on ‘irreplaceable’ Isak and Joelinton fitness

When asked about the importance of Isak to this Newcastle side, Howe agreed he was “irreplaceable”.

“We have a few of those players who are irreplaceable,” he said. “Your top players are so hard to find, so hard to recruit and so hard to develop. So when you have them, you need to treasure them. Of course we’re desperate to keep him as part of our team.”

Howe also said he had spoken with Isak about his future but insists the forward remains focused on performing for Newcastle.

“Yes, I’ve had discussions with him, but that’s not abnormal,” said Howe. “I respect a player’s career and how short it is. Alex has been really good, he’s trained really well and I realise there’ll be noise around him.”

On Joelinton, who was also absent from the matchday squad but in the stands at Celtic, Howe said: “Both players [Joelinton and Isak] are fit. But we just felt they weren’t ready to play.

“Everyone, through pre-season, goes through different challenges. Joe is coming back from a long injury so we need t make sure when he’s introduced he’s ready.”

Howe: We’ll be better for Celtic defeat – but lots of work to do

Reflecting on the performance and defeat to Celtic, Howe was relaxed with where his Newcastle team are, although admitted the goals conceded were poor and there is a lot of work to do to get up to speed for the new season.

“We take a load from that,” he told the club’s media. “That was a really good test for us. The pre-season is designed to really stretch us. We know we’ve got a very challenging season ahead with all the competitions we’re in.

“This was a test that maybe came a little bit too early for us. We’ve had a long week in Austria. Not making excuses, by the way, but the lads came into the game jaded slightly. But it was a great workout for us. Celtic played very well.

“I thought we played well in phases, we created chances and should have scored a couple of goals. Overall I think the team will be better for the game.

“They’re way ahead of us fitness-wise. It’s a useful lesson for us to learn. We usually do that to other teams. The power of our fitness usually shows through in our performances. We were on the receiving end a little bit, reactive rather than proactive. The goals we conceded were the poorest aspect of our performance. It wasn’t a game where we were consistently under pressure we just defended those moments poorly.

“We’ve got a lot of work to do. We know that. We’re in the very early stages of pre-season.”

Analysis: Newcastle loss shows need for signings

Sky Sports News’ Keith Downie at Parkhead:

“Newcastle have been taught a bit of a lesson today. I know Celtic are ahead in terms of pre-season preparation – they’ve been back a couple of weeks – but there has been a gulf between the teams.

“Yes, Newcastle are missing the likes of Isak, Joelinton, Tino Livramento and Lewis Hall, but Celtic have been streets ahead.

“Newcastle fans will be thinking that they need to add more signings – they’ve only added Elanga so far this summer but on this showing, they’ll need a few more.”